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Low-Carb Roast Onion Soup

4.8 stars, average of 4 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.

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Hands-on Overall

Serving size about 1 1/4 cups/ 300 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
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Nutritional values (per serving, about 1 1/4 cups/ 300 ml)

Net carbs7.1 grams
Protein8.8 grams
Fat22.9 grams
Calories274 kcal

Calories from carbs 11%, protein 13%, fat 76%

Total carbs9.9 gramsFiber2.8 gramsSugars4.7 gramsSaturated fat9.6 gramsSodium502 mg(22% RDA)Magnesium36 mg(9% RDA)Potassium561 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 (120 g/ 4.2 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 under 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 1/4 cups/ 300 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 g6 g6.4 g83 kcal
Total per serving, about 1 1/4 cups/ 300 ml
7.1 g8.8 g22.9 g274 kcal

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Dearna Bond
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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 (2)

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!