America’s Test Kitchen
Serves 6 to 8

To make a vegetarian version of this soup, substitute vegetable broth for the chicken broth. If you want to make a vegan version, you’re on your own.

2 tablespoons unsalted butter
2 pounds broccoli, florets roughly chopped into 1-inch pieces, stems trimmed, peeled, and cut into 1/4-inch-thick slices
1 medium onion, roughly chopped (about 1 cup)
2 medium garlic cloves, minced (about 2 teaspoons)
1 ½ teaspoons dry mustard powder
Pinch cayenne pepper
Table salt
3–4 cups water
¼ teaspoon baking soda
2 cups low-sodium chicken broth (see note)
2 ounces baby spinach (2 loosely packed cups)
3 ounces sharp cheddar cheese, shredded (3/4 cup)
1 ½ ounces Parmesan cheese, grated fine (about 3/4 cup), plus extra for serving
Ground black pepper

Heat butter in large Dutch oven over medium-high heat. When foaming subsides, add broccoli, onion, garlic, dry mustard, cayenne, and 1 teaspoon salt. Cook, stirring frequently, until fragrant, about 6 minutes. Add 1 cup water and baking soda. Bring to 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 soup to medium bowl and repeat with remaining soup. Return soup to Dutch oven, place over medium heat and bring to simmer. Adjust consistency of soup with up to 1 cup water. Season to taste with salt and pepper. Serve, passing extra Parmesan separately.