Heat a panini press or a grill pan over medium heat. If using a grill pan, you can press the sandwich with a spatula for even grilling.
Take the ciabatta or focaccia slices and spread the basil pesto evenly on one side of each slice.
Layer the tomato slices over the pesto on one slice, and then layer the mozzarella slices on top of the tomatoes.
Season with a pinch of salt and pepper to enhance the flavors. If using, add a handful of arugula or spinach on top of the mozzarella.
Place the second slice of bread pesto side down on the sandwich.
Brush the outside of both bread slices with olive oil to help them crisp up nicely while grilling.
Place the sandwich in the panini press and cook until the bread is golden brown and the cheese has melted, about 4-5 minutes. If using a grill pan, cook for about 3-4 minutes on each side, pressing down with a spatula.
Remove the panini from the heat, let it cool slightly, then slice in half and serve warm.
Notes
Serve with a side of crispy sweet potato fries or a small salad for a delightful meal. Drizzle a bit more pesto on the plate for added color and flavor!