Low-FODMAP cooking

Onion substitute

The short answer

For the savoury base onion builds under a dish, use a little onion- or garlic-infused oil plus the green tops of spring onions (scallions). The oil carries the aroma and the green tops add fresh allium bite — without the fructans, which sit in the white bulb, not in the oil or the green leaves.

Why onion triggers symptoms

High in Fructans

Onion is one of the most concentrated sources of fructans there is, and the fructans live in the bulb — the part you actually cook with. They aren't fully absorbed in the small intestine, so they travel on to the large intestine where bacteria ferment them into gas. That's the bloating and cramping onion is famous for. The useful quirk for cooks: fructans are water-soluble but not oil-soluble, which is exactly why an infused oil works — the flavour compounds migrate into the fat while the fructans stay behind in the discarded pieces. The green leaves of spring onions, chives, and leeks carry the flavour with almost none of the fructan load.

In short, onion is high in fructans — chain-like carbohydrates (the “oligosaccharides” in FODMAP) that the small intestine can't break down, so gut bacteria ferment them and produce gas. These are all FODMAPs — the fermentable carbs behind most IBS food triggers.

Monash's testing puts even a tablespoon (about 12 g) of onion in the high range, so there's no small "safe" amount to cook down — and onion powder is just as high, gram for gram.

The best low-FODMAP onion substitutes

No single swap does everything oniondoes, so pick the one that fits the job — here’s how each performs, and where it doesn’t.

Onion- or garlic-infused oil

1–2 tbsp infused oil replaces the aromatic base of 1 medium onion

In the kitchen: Warm chopped onion (or garlic) gently in the oil over low heat for a few minutes, then lift out and discard the solids — the fructans stay in the oil-poached pieces, not the oil. Use it as your starting fat for sofrito, curries, dressings, and roasts.
Taste: A rounded, savoury allium background almost indistinguishable from cooking in onion, but with no body or sweetness of its own.
Where it falls short: It's flavour only — no texture, no bulk, and none of the sweet, jammy character of properly caramelised onion. Store-bought infused oils are more reliable than homemade; if you do make your own, refrigerate it and use it within a few days.

Green tops of spring onions (scallions)

The green part of a whole bunch ≈ the fresh bite of half a small onion

In the kitchen: Use only the green leaves — the white bulb is high in fructans like any onion. Add them near the end of cooking, or raw as a finish, so they keep their colour and snap.
Taste: Milder, grassier and fresher than raw onion; closer to a garnish than a base.
Where it falls short: They can't build the deep, sweet, cooked-down foundation of a stew or French onion soup — they're a top note, not a base.

Chives

2–3 tbsp finely snipped chives ≈ a garnish-level hit of raw onion

In the kitchen: Best raw or stirred in off the heat — over baked potatoes, in dips, folded into eggs or dressings.
Taste: Delicate, clean onion flavour without any harshness.
Where it falls short: No cooked depth and no crunch; heat wilts them and dulls the flavour fast.

Asafoetida (hing)

A pinch (about ⅛ tsp) bloomed in hot oil ≈ the savoury depth of onion and garlic together

In the kitchen: A staple of Indian cooking used precisely as an allium stand-in. It must be cooked in hot fat for a few seconds to bloom — raw it's acrid and sulphurous.
Taste: Pungent and umami in the jar; mellows into a savoury, oniony roundness once cooked.
Where it falls short: Many blends are cut with wheat flour, so choose a gluten-free one if you need it. A little goes a very long way — too much is overpowering.

Finely diced carrot (for body and sweetness)

½ cup finely diced carrot replaces the bulk and sweetness of ½ an onion in a braise

In the kitchen: Sweat it slowly in your infused oil to draw out the sugars — it gives soups and stews the soft texture and gentle sweetness a cooked onion would.
Taste: Sweet and earthy rather than savoury; pairs well with the infused oil doing the allium work.
Where it falls short: It brings sweetness and body but none of the allium flavour — use it alongside infused oil, not instead of it.

See the FODMAP data

Every serving threshold below is drawn from published research with the sources shown, so you can see exactly why each food lands where it does.

Onion — see the serving data→Onion powder (also high)→Garlic — the other allium→Leek (white bulb is high)→Shallot (high)→

The bottom line

There's no single swap that does everything an onion does, so the trick is to split the job: infused oil for the savoury base, green spring-onion tops or chives for fresh bite, and a little carrot for sweet body. What you can't fully replace is deeply caramelised onion — the sweet, jammy tangle in a burger or French onion soup. That one, honestly, has no low-FODMAP equal; slow-cooked carrot and a splash of balsamic is the nearest you'll get.

Common questions

What is the best low-FODMAP substitute for onion?

For everyday cooking, onion- or garlic-infused oil is the best all-rounder because it delivers the savoury allium base without the fructans, which don't dissolve into oil. Add the green tops of spring onions or chives for fresh onion bite. Together they cover most of what a raw or sautéed onion contributes.

Why is onion high FODMAP but infused oil isn't?

Onion's fructans are water-soluble, not oil-soluble. When you gently heat onion in oil and then remove the pieces, the flavour compounds migrate into the fat but the fructans stay behind in the discarded onion. That's why the oil carries the taste without the trigger.

Can I just use onion powder instead?

No — onion powder is simply dried, ground onion, so it's just as high in fructans as fresh onion (roughly 1 tablespoon of powder equals a medium onion). It's one of the most common hidden triggers in stock cubes, seasonings and crisps, so it's worth scanning labels for.

More low-FODMAP swaps

Garlic substitute→Honey substitute→Apple substitute→Wheat flour substitute→Milk substitute→Yogurt substitute→Mushrooms substitute→Beans substitute→Mango substitute→

This page is for general cooking and information only and is not medical advice. FODMAP content varies with ripeness, processing, and brand, and serving size is what determines tolerance. Always work with your doctor or a registered dietitian on dietary changes for IBS.