In a small bowl, combine the tuna with lemon juice and season with salt and pepper. Brush the olive oil on the ciabatta. Spread the tuna on the bottom halves of the ciabatta and top with onion slices. Close the panini.
Set a large cast-iron skillet over moderately high heat. Arrange the panini in the skillet and weight them down with a small pan. Cook the panini until the outside is crisp and the filling is heated through, 1 or 2 minutes per side. Cut the panini in half and serve at once.
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