Cincinnati-style steak hoagie with mushroom gravy

Ingredients

• 3 tablespoons butter
• 1/2 onion, diced
• 10 ounces mushrooms, sliced
• 2 cloves garlic, finely minced or pressed
• 2 tablespoons all-purpose flour
• 2 cups beef broth
• 1 cup milk
• 1/2 teaspoon salt
• 1/2 teaspoon ground black pepper
• 1/4 teaspoon MSG (optional)
• 6-to-8-inch bread roll, sliced
• 2 tablespoons softened butter
• 2 cloves garlic, finely diced or pressed
• 1 teaspoon parsley, finely chopped
• 1 teaspoon grated parmesan cheese
• 5 to 6 ounces ground steak or ground beef
• salt
• ground black pepper
• garlic bread roll (from above)
• mushroom sauce (from above)
• ground steak/beef patty (from above)
• mozzarella cheese, shredded
• dill pickle slices (optional)
• thinly sliced onion (optional)