Thai Coconut Soup (Tom Kha)

Tom Kha is a Thai coconut soup, and this low-FODMAP version portions the coconut milk to a tested-safe amount, uses oyster mushrooms (the one low-FODMAP mushroom), and builds savory depth from garlic-infused oil and scallion green tops instead of onion and garlic bulbs.

Prep 15 min

Cook 25 min

Serves 4

Gluten-freeDairy-free

Ingredients

• 2 tablespoons garlic-infused olive oil
• 2 stalks lemongrass, tough outer layers removed, bruised and cut into 2-inch pieces
• 1 thumb-sized piece fresh galangal (about 6 slices), or fresh ginger
• 4 makrut (kaffir) lime leaves, torn (optional, but authentic)
• 4 cups (1 liter) low-FODMAP chicken broth
• 1 1/2 cups (350 ml) canned coconut milk (this keeps each serving under about 100 ml, roughly 1/3 cup; check the Monash app for your tested amount)
• 400 g (14 oz) boneless skinless chicken thighs or breast, thinly sliced
• 200 g (7 oz) oyster mushrooms, torn into strips (the only mushroom tested low-FODMAP)
• Green tops of 2 scallions, sliced (save the white bulbs for something else, they are high-FODMAP)
• 2 to 3 tablespoons fish sauce (check the label for added onion or garlic)
• 2 to 3 tablespoons fresh lime juice
• 1 to 2 teaspoons cane sugar or maple syrup
• 1 red chili, sliced (optional)
• Small handful fresh cilantro, chopped, to garnish

Instructions

Build the aromatic base

1. Warm the garlic-infused oil in a large pot over medium heat. Add the lemongrass, galangal, and lime leaves and stir for about 1 minute, until fragrant.
2. Pour in the chicken broth. Bring to a gentle simmer and cook for 10 minutes so the aromatics infuse the liquid.

Add coconut milk and chicken

1. Lower the heat and stir in the coconut milk. Keep the soup at a bare simmer, since a hard boil can split it.
2. Add the sliced chicken and cook for 5 to 7 minutes, until cooked through to 165°F (74°C).
3. Add the oyster mushrooms and simmer for 3 to 4 minutes, until tender.

Season and finish

1. Turn off the heat. Stir in the fish sauce, lime juice, and sugar. Taste and adjust so the broth reads sour, salty, and lightly sweet.
2. Lift out the lemongrass, galangal, and lime leaves if you like. They flavor the broth but are not meant to be eaten.
3. Ladle into bowls and top with scallion green tops, chili, and cilantro.

Tips & Substitutions

• Make it vegan. Swap the chicken for firm tofu (silken tofu is not tested low-FODMAP) and use a low-FODMAP vegetable broth with a splash of tamari in place of fish sauce.
• No galangal on hand. Fresh ginger is a close, easy substitute and is also low-FODMAP. Use a similar amount.
• Stick to oyster mushrooms. Button, cremini, portobello, and dried porcini are high in polyols. Oyster mushrooms are the tested-safe choice here.
• Coconut cream is not coconut milk. Coconut cream is more concentrated and has a smaller safe serving, so if you swap it in, cut the amount and check the Monash app.
• Fish sauce labels vary. Some blends add garlic or onion. A plain fish sauce (fish, salt, sugar) is the safest pick.
• Add a low-FODMAP vegetable. A few halved cherry tomatoes or a handful of baby spinach stir in well without adding much FODMAP load.

Why This Works

• Garlic-infused oil, no bulbs. Fructans do not dissolve into oil, so the oil carries the garlic flavor without the FODMAPs that make garlic cloves a problem.
• Scallion greens, not whites. The green tops of scallions are low in fructans, while the white bulbs are high, so using only the greens keeps the onion note safe.
• Coconut milk is portioned, not omitted. Coconut milk is low-FODMAP in a controlled amount, so diluting it with broth keeps each bowl under the tested serving.
• Oyster mushrooms are the safe mushroom. Most mushrooms carry mannitol, a polyol, while oyster mushrooms test low and still give the soup its earthy body.

Storage

Store cooled soup in an airtight container in the fridge for up to 3 days. Reheat gently over low heat so the coconut milk does not split, and avoid a rolling boil. You can freeze it for up to 1 month, though the coconut milk may turn slightly grainy on thawing, so stir well as it reheats. Keep servings to one bowl to stay within the coconut milk portion.

Not sure about an ingredient? The FODMAP Foods app rates 1,000+ foods low, moderate, or high FODMAP, with the safe portion for each, so you can cook with confidence.