Slice the lemon into thin rounds. Slice the courgette, cut the red pepper into strips, and finely slice the red onion.
Assembling the papillotes: Prepare four large sheets of baking parchment. Place some slices of courgette, red pepper strips, and red onion slices in the centre of each sheet.
Place the tuna on top of the vegetables. Sprinkle with the chopped garlic, salt and pepper.
Cooking: Seal the edges of the parchment to create the papillotes. Place the papillotes on a baking tray and bake for approximately 20 minutes at 180°C.
Presentation: Serve the papillotes directly on the plates. Open them carefully to release the fragrant aromas.