8 cups water
3 Tbs., plus 1 tsp. kosher salt
2 broccoli crowns, cut into 2-inch florets
4 Tbs. unsalted butter
1/4 cup all-purpose flour
1 cup finely chopped onion
2 cups whole milk, room temperature
1 tsp. dry mustard
1 pinch cayenne pepper
1 pinch freshly grated nutmeg
1 cup freshly grated aged cheddar cheese
1/2 cup mascarpone cheese
1/2 cup freshly grated Parmesan cheese
1/2 cup dry bread crumbs
Preheat the oven to 425°F. Line a baking sheet with a kitchen towel.
Put the water in a large stock pot over high heat and bring to a rolling boil. Add three tablespoons of the salt and stir to dissolve. Add the broccoli florets and cook until just tender, about three minutes. Drain and spread out on the prepared baking sheet.
Heat the butter in a small saucepan over medium heat. Whisk in the flour and whisk gently for three to four minutes, until the flour just begins to color. Add onions and cook for three to four minutes longer, until the onions have softened.
Stir in the milk, the remaining teaspoon of salt, the mustard, cayenne, and nutmeg. Bring to a simmer, stirring, and the mixture will thicken. Adjust the seasoning to your liking and remove from heat.
Stir in the cheddar in three additions to ensure the cheese sauce doesn’t get grainy. Whisk in the mascarpone.
Place the broccoli florets in a nine- by nine-inch baking dish. Cover them with the cheese mixture. Top with the grated Parmesan and bread crumbs and bake until golden brown, about 30 minutes.
Let cool for five to 10 minutes before serving.