In a large pot or dutch oven, heat butter over medium high heat. Add broccoli, onion, garlic, mustard, cayenne and 1 teaspoon salt. Stir frequently and cook until fragrant, about 6 minutes.
Add 1 cup water and baking soda and bring to a simmer. Cook until broccoli is very soft, about 20 minutes.
Add broth and 2 cups water and increase heat to medium high. Bring to a simmer and cook another 5 minutes. Add cheeses and blend soup with an immersion blender (alternatively, you can transfer soup to a blender and blend in two batches).
Adjust consistency of soup to your liking with remaining water (I found it didn’t need any more). Season to taste with salt and pepper.
Notes
Serves 8 as a side or appetizer. Each serving has 6.48 g NET CARBS.Food energy: 217kcal Total fat: 12.88g Calories from fat: 115 Cholesterol: 41mg Carbohydrate: 9.68g Total dietary fiber: 3.20g Protein: 13.63g