If garlic is the #1 FODMAP trap, onion is #2 by a hair. The bulb of every allium — white, yellow, red, sweet, shallots, the white part of scallions, the bulb of leeks — is high in fructans. Cooking, slicing, soaking, none of those reduce the fructans enough to help.
The good news: the green tops of these same plants are low FODMAP, and most of the onion-flavor heavy lifting can be done with them.
What’s high FODMAP vs low FODMAP
HIGH FODMAP (avoid):
- Onion (any color, raw or cooked)
- Shallots (raw or cooked)
- Garlic
- White part of scallions / green onions / spring onions
- White bulb of leeks
- Onion powder, onion salt, dehydrated onion
LOW FODMAP (safe):
- Green tops of scallions / spring onions
- Green tops of leeks
- Chives (Allium schoenoprasum)
- Garlic-infused oil (see our garlic article)
- Asafoetida (hing) — cooked in oil
The cooking rule: anything below the soil line of an allium plant is fructans. Anything above (green leaves) is low FODMAP.
The 5 best swaps
1. Green parts of scallions / spring onions
The single most useful onion substitute. Use freely. Sharp, mild onion flavor. Chop fine for stir-fries, soups, garnishes.
When buying scallions, use the green tops for cooking, discard or compost the white bulbs (or save them for vegetable scraps that you don’t eat).
2. Green tops of leeks
Same principle. The green leaves above the white bulb part are low FODMAP. Slice thinly and add to soups, stews, sautés. Mild sweet onion flavor when cooked.
3. Chives
Most delicate of the three. Better as finishing herb than as a base for cooking. Sprinkle on eggs, baked potato, soup, salads.
4. Onion-infused oil (DIY)
Same trick as garlic-infused oil:
- Heat oil with sliced onion 5-10 minutes
- Remove and discard the onion solids
- Use the oil for cooking
You get onion flavor without water-soluble fructans. Make in small batches, refrigerate, use within 1 week.
5. Asafoetida (hing)
Mentioned in the garlic article — it provides both garlic and onion flavor when cooked in oil. A pinch goes a long way. Especially useful for South Asian and Middle Eastern dishes.
A practical 7-day kitchen reset
Day 1: Throw out (or move out of immediate reach) onions, shallots, onion powder. Move them to a “rebuild later” shelf.
Day 2: Buy scallions (use green parts only), chives, garlic-infused olive oil, asafoetida (optional).
Day 3: Cook one familiar dish using only the substitutes. Note the flavor: you’ll likely find the dish tastes 80-90% of the original. The missing 10-20% is mostly a matter of habit/expectation.
Day 4-7: Build out 5-6 reliable recipes with the new swaps. Once you have a rotation, the diet becomes much easier.
Common dish translations
| Original | IBS-safe version |
|---|---|
| Sautéed onions to start a dish | Scallion greens + garlic-infused oil |
| French onion soup | Make with leek greens + scallion greens (very different but workable) |
| Onion in chili / curry | Scallion greens + asafoetida |
| Salsa with raw onion | Skip onion; rely on tomato + lime + cilantro + chives |
| Onion in tuna salad | Chives + pickled cucumber for the crunchy element |
| Onion rings | Generally not possible; consider zucchini fritters as a substitute |
Where onion hides
Like garlic, onion is in many pre-made foods:
- “Natural flavors” — onion powder is one of the most common
- All commercial stocks and broths — onion is usually #2 or #3 ingredient
- Bouillon cubes — high onion content
- Salad dressings, marinades, pasta sauces — assume onion until labels confirm otherwise
- Most chips and crackers with “savory” flavoring
- Most soups in cans / packets
- Burger patties at restaurants — often have onion mixed in
- Hummus, baba ganoush — frequently
- Sausages, breakfast meats
Low-FODMAP-certified products are the workaround in 2026 — Monash certified products explicitly avoid onion.
What about onion’s relatives?
Spring onions / scallions — green tops yes, white bulbs no Leeks — green tops yes, white bulbs no Shallots — entire bulb is high FODMAP Pearl onions — high FODMAP Red onions — same as yellow, high FODMAP regardless of color Caramelized onions — cooking doesn’t break down fructans; still high FODMAP
The “hidden in restaurants” problem
Eating out is where onion gets you. Almost every savory restaurant dish starts with sautéed onion. The workarounds:
- Ask if a dish can be made “without onion” — many can; some are based around it
- Stick to grilled / roasted protein + plain rice / potato + vegetable when uncertain
- Asian restaurants: avoid sauces that include onion (most stir-fries, curries); plain steamed rice + protein + steamed greens is usually safe
- Mexican: ask for no onion in salsa, beans, fillings; pico de gallo will have onion by default
Carry an “I have a food intolerance to onion and garlic” card if traveling internationally — most kitchens take this seriously even without knowing what IBS means.
Bottom line
Onion isn’t optional in most cooking, but substituting with scallion greens + garlic-infused oil + occasional asafoetida covers 95% of cases. The first week feels weird. By week 3, you don’t miss it.
Combined with our garlic substitution guide, most IBS patients can eat 90%+ of the savory dishes they ate before low-FODMAP — they just cook them differently.