4 tablespoons unsalted butter, plus more for the pan
1/2 cup finely chopped yellow onion
3 tablespoons all-purpose flour
1 1/2 cup hot milk
1 1/2 cups grated sharp Cheddar (about 6 ounces)
2 heads broccoli, cut into 2-inch pieces, quickly steamed (see Cook's Note)
1 1/2 cups crushed butter crackers (from about 1 sleeve)
To steam, put the broccoli in a microwave-safe bowl. Add a little water to the bottom of the bowl and top with a microwave-safe plate. Microwave on high for 2 to 3 minutes, until bright green and crisp-tender.