Step 1

Preheat the oven to 350 degrees F. Butter a 2-quart baking dish or gratin dish.

Step 2

Melt 2 tablespoons butter in a medium saucepan over medium-high heat. Add the onions and 1 teaspoon salt and cook, stirring frequently, until the onions are translucent, about 4 minutes. Stir in the flour and stir to coat the onions. Cook for 2 minutes. Remove from the heat and whisk in the hot milk. Return to the heat and bring to a simmer. Stir in the cheese by the handful. Add salt to taste. Add the broccoli to the cheese sauce and pour into the prepared dish.

Step 3

Melt the remaining 2 tablespoons butter in a small skillet and stir in the cracker crumbs. Cover the broccoli casserole with the buttered cracker crumbs. Bake until bubbly and lightly browned, about 20 minutes.