Preheat the oven to 350°F. Spray a 11x7 inch baking dish with cooking spray. Set aside. (I used an oblong pan but any 2 quart baking dish will do)
Steam the broccoli, slightly under cooking. Cool.
Prepare Sauce: In a medium saucepan, melt 6 tablespoons of butter. Saute the minced onion in the butter for 2 minutes just until translucent.
Sprinkle the flour over the butter. Add the dry mustard , garlic salt and black pepper. Stir until it the flour is absorbed by the butter. Cook stirring constantly for 1 minute.
Gradually add the milk, whisking constantly over medium-high heat. When the sauce begins to boil, lower the heat and simmer gently for 5 minutes.
Remove from the heat and add 2 cup shredded cheddar cheese, reserving 1 cup for the top. Stir until the cheese has blended with the sauce.
Add the steamed broccoli and using a large spatula, gently mix until combined. Pour into the prepared baking dish and sprinkle the remaining 1 cup shredded cheddar cheese on top.
Melt the remaining 2 Tbsp melted butter and mix with 2/3 panko breadcrumbs, 1/4 grated Parmesan cheese and 1/4 tsp garlic salt. Sprinkle on top.
Bake for 35-40 minutes until golden and bubbly. Serve immediately.