Broccoli au Gratin

Name: Broccoli au Gratin
Ingredients: Cooking spray or butter, for coating the baking dish; 1 1/2 pounds broccoli crowns (about 3 medium), broccoli florets, or frozen broccoli; 1/2 medium yellow onion; 8 ounces sharp cheddar cheese (2 cups shredded), preferably white; 5 tablespoons unsalted butter; 3 tablespoons all-purpose flour; 1/2 teaspoon dry mustard, or 1 1/2 teaspoons Dijon mustard; 2 cups whole or 2% milk; 3/4 teaspoon kosher salt; 1/3 cup panko breadcrumbs; 2 tablespoons grated Parmesan cheese