First, heat a large pot over medium heat. Add oil and cook onion and leek for about 3-4 minutes.
Then, add garlic and cook for about 1 minute, stirring every now and then. Do not let brown! Transfer about half of the veggies to the blender; reserve.
Next, add broth and cauliflower to the pot and let cook over high heat for about 8-10 minutes or until tender. Transfer all the cauliflower and 1 cup of the broth to the blender. Add salt, pepper, cayenne pepper, and heavy cream. Blend with half of the reserved cooked veggies until smooth. Pour back into the pot.
Add the fish and potatoes. Let cook over medium-high heat for about 5-6 minutes. Remove pot from the heat and stir in lemon juice.
Last, garnish fish soup with fresh dill or chives. Serve by itself or with a hard-crusted bread.
