In a food processor, combine fresh basil leaves, garlic cloves, walnuts, grated Parmesan cheese, salt, and pepper. Pulse until finely chopped, then drizzle in olive oil while blending until smooth and creamy.

Take two slices of crispy sourdough bread and layer with slices of fresh mozzarella, ripe tomato, and roasted red peppers. Spread the homemade pesto over all ingredients.

Heat a grill pan over medium heat. Place the assembled sandwich in the pan and cook for 3-5 minutes on one side. Flip and toast the other side until crispy and cheese is melted.

Remove from the grill pan, let cool for a minute, then slice in half to serve warm.