How to Make It
Coat the bottom of a large skillet with olive oil and heat until shimmering. Season the eggplant slices with salt and pepper and cook over moderately high heat until golden, 6 to 7 minutes per side. Top each eggplant slice with a slice of cheese and cook until the cheese begins to melt, 1 to 2 minutes. Transfer to paper towels to drain. Wipe out the skillet and add more oil. Repeat with the remaining eggplant and cheese.
Arrange the tomatoes and basil on the bottom halves of the focaccia and season with salt and pepper. Top with the eggplant slices and close the panini.
Set a large cast-iron skillet or griddle over moderately high heat. Arrange the panini in the skillet and weight them down with a smaller pan. Cook the panini until the outside is crisp and the filling is heated through, 3 minutes per side. Cut the panini in half and serve at once.