Broccoli-Cheese Casserole

Steam broccoli:

Place broccoli and water in a large microwavable bowl. Cover loosely with plastic wrap, or a microwave-safe lid, vented, and microwave on HIGH until tender, about 8 minutes. Set aside.

Make sauce:

Meanwhile, melt 3 tablespoons of the butter in a Dutch oven or large saucepan over medium-high. Add onion, and cook, stirring occasionally, until softened, about 5 minutes. Add garlic, and cook, stirring occasionally and reducing heat if needed to prevent burning, 1 minute. Sprinkle flour evenly over onion mixture, and cook, stirring constantly, 1 minute. Gradually whisk in milk, and bring to a simmer over medium-high heat. Cook, stirring constantly with a whisk, until thickened, about 2 minutes. Reduce heat to medium, and whisk in mayonnaise, mustard, salt, and pepper until smooth.

Add broccoli and cheese to sauce:

Add cheese, and stir until melted and smooth. Add broccoli, and stir to coat.

Add casserole to baking dish:

Transfer the broccoli and cheese mixture to a lightly greased 13- x 9-inch broiler-proof baking dish. If you're baking now, go to the next step. If you're making this dish ahead, see the tip below.

Bake casserole:

Preheat oven to 350°F. Cover with aluminum foil. Bake, covered, until hot and bubbly, about 45 minutes.

Make casserole topping:

Place remaining 1 tablespoon butter in a medium-size microwavable bowl, and microwave on HIGH until melted, about 15 seconds. Add crackers and parsley, and stir to combine. Remove the aluminum foil from the casserole and sprinkle evenly over casserole. Increase oven temperature to broil on HIGH, and broil casserole until top is golden brown, about 2 minutes. Let stand 10 minutes before serving.