To assemble, smear marinara sauce on half the pieces of focaccia, then top with basil and mozzarella. Smear the pesto on the other pieces and assemble the sandwiches.

Heat a large skillet over a medium-high flame. Melt 1 tablespoon of butter in the preheated skillet. Once melted, place 1 or 2 sandwiches in the pan (depending on space) and fry on both sides until the cheese melts, about 4 to 6 minutes on each side. (If the bread begins to burn before the cheese melts, reduce the heat and cover the skillet.) Add 1 tablespoon butter in between every sandwich. Serve hot.