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Low-Carb Roast Onion Soup

4.7 stars, average of 67 ratings

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This keto riff on French onion soup uses cauliflower to lower the carb content. By roasting the cauliflower and onion together, the onions flavour is intensified and the resulting soup is rich and flavoursome.

You can substitute the fresh onion and cauliflower here for frozen cauliflower and onion, or alternatively if you want to freeze the soup, you can double or triple the quantities and portion out to freeze.

This low-carb soup is delicious served with some grated Swiss cheese melted on top, or even with a piece of toasted keto bread.

Hands-on Overall

Serving size about 1 1/3 cups/ 320 ml

Allergy information for Low-Carb Roast Onion Soup

✔  Gluten free
✔  Egg free
✔  Nut free
✔  Nightshade free
✔  Pork free
✔  Avocado free
✔  Coconut free
✔  Fish free
✔  Shellfish free
✔  Beef free

Nutritional values (per serving, about 1 1/3 cups/ 320 ml)

Net carbs7.1 grams
Protein8.5 grams
Fat22.5 grams
Calories269 kcal
Calories from carbs 11%, protein 13%, fat 76%
Total carbs9.9 gramsFiber2.8 gramsSugars4.7 gramsSaturated fat9.4 gramsSodium498 mg(22% RDA)Magnesium35 mg(9% RDA)Potassium560 mg(28% EMR)

Ingredients (makes 6 servings)

  • 2 large brown onions (300 g/ 10.6 oz)
  • 1/2 large head cauliflower (450 g/ 1 lb)
  • 4 cloves garlic, still in skin
  • 4 tbsp extra virgin olive oil or ghee (60 ml)
  • 1/2 tsp sea salt
  • 1/4 tsp cracked pepper
  • 4 cups vegetable stock or chicken stock (960 ml/ 32 fl oz)
  • 1 cup water (240 ml/ 8 fl oz)
  • 1 cup grated gruyere or cheese of choice (115 g/ 4 oz)


  1. Heat oven to 190 °C/ 375 °F (conventional), or 170 °C/ 340 °F (fan assisted). Chop the cauliflower into florets and slice the onion. Low-Carb Roast Onion Soup
  2. Place in a roasting dish and toss with the garlic cloves, olive oil, salt and pepper.
  3. Bake for about 45 minutes, stirring half way through, until the vegetables are nicely browned but not burnt.
    Low-Carb Roast Onion Soup
  4. Remove from the oven and add the cauliflower and onion to a pot with the stock and water.
  5. Squeeze the garlic from the skins into the pot. Bring to a simmer and cook for about 10 minutes.
    Low-Carb Roast Onion Soup
  6. Remove from heat and serve topped with grated cheese. Store in the fridge up to 5 days or freeze for up to 3 months. Low-Carb Roast Onion Soup

Ingredient nutritional breakdown (per serving, about 1 1/3 cups/ 320 ml)

Net carbsProteinFatCalories
Onion, brown (yellow), raw
3.2 g0.5 g0.1 g18 kcal
Cauliflower, fresh
2.2 g1.4 g0.2 g19 kcal
Garlic, fresh
0.6 g0.1 g0 g3 kcal
Olive oil, extra virgin
0 g0 g9 g80 kcal
Salt, sea salt
0 g0 g0 g0 kcal
Pepper, black, spices
0 g0 g0 g0 kcal
Vegetable stock
1 g0.8 g7.2 g72 kcal
Water, still
0 g0 g0 g0 kcal
Cheese, Swiss , Gruyère
0.1 g5.7 g6 g78 kcal
Total per serving, about 1 1/3 cups/ 320 ml
7.1 g8.5 g22.5 g269 kcal

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Dearna Bond
Creator of

Dearna Bond

Dearna is a passionate foodie and food photographer, and loves sharing her knowledge and enthusiasm for both via her food blog and online photography courses.

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Comments (4)

This looks so good, and I’d like to make it. How would it be if I left the cheese out?

His Judy, I'd say quite plain as the cheese is one of the key ingredients.

Smelled so delicious in my house, I used my immersion blender so my husband could eat without his teeth in! He and my daughter and of course myself loved it! I will be making this soup again!! Thank you for sharing

Glad you enjoyed Juli!