How to Make It
Melt the butter in a large pot over medium-high heat. Sprinkle with the flour and cook, whisking, for 30 seconds. Slowly whisk in the milk, 1 teaspoon salt, and ¼ teaspoon pepper.
Simmer, whisking occasionally, until thickened, 10 to 12 minutes. Stir in the broccoli and cook until crisp-tender, 4 to 8 minutes. Stir in 1 cup of the Cheddar. Heat broiler.
Transfer the broccoli mixture to an 8-inch or other 2-quart baking dish and top with the remaining ¼ cup of cheese. Broil until the top is golden, 2 to 4 minutes.