- 2 tablespoons unsalted butter
- 2 pounds fresh broccoli
- 1 medium onion, roughly chopped (about 1 cup)
- 2 medium garlic cloves, minced (about 2 teaspoons)
- 1 1/2 teaspoons dry mustard powder
- pinch cayenne pepper
- 3-4 cups water
- 1/4 teaspoon baking soda
- 2 cups low-sodium chicken broth
- 2 ounces baby spinach
- 3 ounces sharp cheddar cheese, shredded (about 3/4 cup)
- 1 1/2 ounces Parmesan cheese, grated fine, plus extra for serving
- ground black pepper
- Chop broccoli florets into roughly 1-inch pieces. Trim the stems, peel, and cut them into 1/4-inch-thick slices.
- Heat butter in large Dutch oven over medium-high heat. When foaming subsides, add broccoli, onion, garlic, mustard, cayenne, and 1 teaspoon salt. Cook, stirring frequently, until fragrant, about 6 minutes. Add 1 cup water and baking soda. Bring to a simmer, cover, and cook until broccoli is very soft, about 20 minutes, stirring once during cooking.
- Add broth and 2 cups water and increase heat to medium-high. When mixture begins to simmer, stir in spinach and cook until wilted, about 1 minute. Transfer half of soup to blender, add cheddar and Parmesan, and process until smooth, about 1 minute. Transfer blended soup to a medium bowl and repeat with remaining soup. Return soup to Dutch oven, place over medium heat and bring to a simmer. Adjust consistency of soup with up to 1 cup of water. Season to taste with salt and pepper. Serve, passing extra Parmesan separately.