Step 1

Preheat oven to 350 degrees F (175 degrees C). Grease a 9x12-inch baking dish.

Step 2

Place broccoli and onion into a saucepan, pour in water, and bring to a boil. Reduce heat to low, cover, and cook until broccoli turns bright green and starts to become tender, 8 to 10 minutes.

Step 3

Whisk flour and 1/4 cup evaporated milk together in a bowl until smooth. Stir remaining evaporated milk and salt into broccoli. Gently stir flour mixture into broccoli mixture and bring to a simmer; cook until thickened, 2 to 3 minutes. Remove from heat and stir in sharp Cheddar cheese, mixing until cheese has melted into the sauce.

Step 4

Cook and stir bread crumbs with butter in a small skillet over medium heat until butter has melted and crumbs begin to give a toasted fragrance, 1 to 2 minutes. Remove from heat.

Step 5

Transfer broccoli mixture into the prepared baking dish and sprinkle crumbs atop casserole.

Step 6

Bake in the preheated oven until bubbling and browned, about 30 minutes.