Peel and finely chop the onion. Wash, halve and seed the tomato and also cut into fine cubes. Mix lemon juice, onions, salt, pepper and olive oil. Drain the tuna and stir into the marinade with the diced tomatoes.
Remove the seeds from the melon and cut out 12-16 balls with a ball cutter. Wash the basil, dab dry and pluck the leaves from the stalks. Spread baguette slices with salad cream. Cover 4 slices with Parma ham and melon balls.
Season with pepper and garnish with basil. Spread on the remaining slices of tuna and tomato salad. Arrange canapés on 4 plates and garnish with some basil leaves.