Generously grease an 8 x 8-inch baking dish. Preheat oven to 375 degrees F.
Steam broccoli until crisp-tender, 5 minutes from fresh; 10 minutes from frozen. Add to prepared dish; set aside.
In a medium saucepan set to medium heat, melt butter. Whisk in flour and cook, stirring, for 1 minute. Add milk, salt, garlic powder, onion powder and nutmeg. Bring to a boil while whisking constantly. Reduce heat and simmer, stirring occasionally, until sauce is thickened, about 5 minutes. Remove from heat and stir in cheddar.
Pour sauce evenly over broccoli. Sprinkle with French fried onions and crumbled croutons.
Bake until lightly browned and bubbly, about 30 minutes. Tent with foil if browning too quickly. Serve hot.