Broccoli Cheese Soup

adapted from America's Test Kitchen

2 Tbs unsalted butter
2 lbs broccoli, florets chopped into 1-inch pieces and stems peeled and cut into 1/4-inch thick slices
1 medium onion, roughly chopped (about 1 cup)
2 medium cloves of garlic, minced (about 2 tsp)
1 1/2 tsp dry mustard powder
pinch cayenne pepper
1 tsp Kosher salt
3 - 4 cups water
1/4 tsp baking soda
2 cups low-sodium chicken broth or stock
2 oz baby spinach (about 2 cups loosely packed)
3 oz sharp cheddar cheese, shredded
1 1/2 oz Parmesan cheese, grated
Salt and black pepper to taste

Heat butter in a large Dutch oven over medium-high heat.  When foaming subsides, add onion and sauté until softened. Add broccoli, garlic, dry mustard, cayenne and 1 tsp salt.  Cook, stirring frequently, until fragrant (about 6 minutes).  Add 1 cup of water and baking soda.  Bring to a simmer, cover, and reduce heat to low.  Cook until the broccoli is extremely soft and falling apart (overcooked).  Stir occasionally.

Add broth and 2 cups of water and increase heat to medium-high.  When the mixture reaches a hard simmer, add the spinach and cook until it is wilted, one or two minutes at most.  Stir in the cheese until melted.  With an immersion blender (or working in batches with a regular blender), puree the soup until it is smooth.  Adjust the consistency of the soup as necessary with up to one cup of water.  Bring back to temperature; season to taste with salt and pepper.  Serve, passing additional Parmesan separately.

Note:  This soup can be made vegetarian by substituting vegetable broth for the chicken broth.