Preheat the grill or a non-stick skillet over medium heat. Slice the sourdough bread, fresh mozzarella, and roasted red bell peppers. Rinse and dry the fresh spinach and basil.

In a bowl, combine balsamic vinaigrette, chopped basil, garlic powder, salt, and pepper. Mix until well combined.

Lay 4 slices of sourdough bread on a clean surface. Layer with fresh mozzarella, roasted red bell peppers, and spinach. Drizzle the balsamic mixture over the fillings. Top with the remaining 4 slices of bread.

Brush the outside of each sandwich with olive oil. Place sandwiches on the preheated grill or skillet. Cook for 4-5 minutes on each side until the bread turns golden brown and the cheese melts.

Remove the sandwiches from heat and let them cool briefly. Slice each sandwich in half and serve with additional balsamic vinaigrette on the side, if desired.