1. Spread mayonnaise evenly on 1 side of each sourdough bread slice.
2. Place 4 bread slices, mayonnaise sides down, on wax paper, and layer each with 1 1/2 mozzarella cheese slices and 1 1/2 Cheddar cheese slices. Top with remaining bread slices, mayonnaise sides up.
3. Cook sandwiches, in batches, on a hot griddle or in a nonstick skillet over medium heat 3 to 5 minutes on each side or until golden brown and cheese is melted.
Note: Sandwiches can be cooked in a preheated panini press 2 to 3 minutes or until golden brown.
Sweet-and-Spicy Grilled Cheese: Substitute 8 ounces Havarti cheese slices for mozzarella and Cheddar. Layer cheese as directed; spread cheese evenly with 1/2 cup red pepper jelly. Proceed as directed.
Mexi-Melt Grilled Cheese: Substitute 8 ounces Monterey Jack cheese with peppers slices for mozzarella and Cheddar. Layer cheese as directed; spread cheese evenly with 1/2 cup refried beans, and sprinkle with 1/4 cup chopped fresh cilantro. Proceed as directed.
Apple-Cheddar Grilled Cheese: Place 1/2 cup chopped walnuts in a single layer in a shallow pan. Bake at 350° for 8 to 10 minutes or until lightly toasted, stirring occasionally. Substitute 4 ounces Cheddar slices for mozzarella. Layer cheese as directed; top evenly with 1 thinly sliced Gala apple and toasted walnuts. Sprinkle with pepper to taste. Proceed as directed.