Salisbury Steaks:

1. In a large bowl combine bread crumbs, beaten egg, onion powder, garlic powder, salt and pepper together. Crumble ground beef over the top and mix well. Form into six patties.
2. Heat coals in a charcoal grill until they turn orange and ash over.
3. Grill the burger patties over direct heat, with the lid closed, until cooked to desired doneness, flipping once.
4. Prepare salisbury steak brown gravy as instructed below and add grilled patties. Let patties simmer in the gravy, covered, for about 15-20 minutes.

Gravy:

1. Make roux: Add butter and flour to a large pot and stir continuously until a roux is formed, about 2 minutes.
2. Add beef broth and spices: Slowly pour in half the beef broth while stirring vigorously with a whisk. This will help prevent lumps. Once smooth and lump-free, add the rest of the beef broth, onion powder, garlic powder, beef bouillon base (if using), and bay leaf (if using).
3. Thicken: Let the gravy boil until thick and smooth, about 3 minutes. Taste and season with salt and pepper. Add mushrooms.