Grilled Cheese and Pepperoni Sandwich

Ingredients (10)

• 6 tablespoons butter, softened, divided
• 8 slices sourdough bread
• 1/2 cup shredded sharp white cheddar cheese
• 1/2 cup shredded Monterey Jack cheese
• 1/2 cup shredded Gruyere cheese
• 3 tablespoons mayonnaise
• 3 tablespoons finely shredded Manchego or Parmesan cheese
• 1/8 teaspoon onion powder
• 24 slices pepperoni
• 4 ounces Brie cheese, rind removed and sliced