How to Make It
In a small bowl, combine the tuna with the lemon juice and season with salt and pepper. Brush the olive oil on the bread or ciabatta. Spread the tuna on 4 slices of the bread and top with onion slices. 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, 1 to 2 minutes per side. Cut the panini in half and serve at once.