Put the basil leaves in a blender or food processor. Add the walnuts, garlic, Parmesan cheese, then season with some salt and pepper. As the ingredients are blending, pour gradually the olive oil.
To make the sandwich:
Spread the pesto sauce on the 4 slices of bread. Put 2 slices of bread on your panini maker or in the oven for 2 minutes.
Next, lay the Mozzarella cheese, tomato slices, roasted red peppers on top and let your panini open.
Once the cheese starts to melt and your veggies to soften, add the 2 remaining slices on top, then close the panini maker for about 3 minutes.
Transfer the grilled sandwiches to a plate, slice in half and enjoy immediately.