In a blender, pulse baby spinach, basil, garlic, salt, pepper until finely chopped. With the blender still running, add a squeeze of lemon juice and gradually enough oil to form a smooth and thick consistency. Lastly, add the Parmesan. Taste and add more salt or pepper if needed.
Preheat oven to 380 F.
Sprinkle each Tilapia filets with some salt, pepper, Italian seasoning and a little squeeze of lime. Heat a pan with some oil in it. When it is warm put the Tilapia and let the fish cook until lightly brown on each side. Put aside.
Cut the bread in half horizontally. On each slice of bread spread some spinach basil pesto. On the bottom slice, put slices of onions, a little bit of Swiss cheese, some Tilapia filets, and the remaining bread slice on top. Press a little bit.
Put the four sandwiches on a stone pizza or cookie sheet and cook for about 15 min.
Remove from the oven and cut them in half. I like to serve them with a salad on the side.