In a food processor, pulse together garlic cloves and walnuts until finely chopped. Add basil leaves, vegetarian Parmesan, salt, and pepper. While blending, drizzle in olive oil until smooth.

Slice tomatoes and mozzarella into even rounds. Arrange them on a platter with spinach. Drizzle with olive oil and balsamic reduction, and sprinkle with seasonings.

Spread pesto on one side of each ciabatta slice. Layer tomatoes, spinach, and mozzarella on one slice, then top with the other slice, pesto side down.

Grill or toast the sandwich on a skillet for 3-4 minutes per side until golden brown. Alternatively, cut into quarters for serving cold.