1. Take a rectangular mold measuring 11x33 cm and line it with 2 large sheets of plastic wrap so you can wrap the sandwich log tightly later. Drain the tuna cans well. Place the tuna in a bowl and add the corn, carrot, pepper, spring onions, olives, lettuce, parsley, lemon juice, salt, pepper, and 2/3 of the mayonnaise. Mix well.
2. Spread a little mayonnaise on one side of 3 slices and place them at the bottom of the mold with the spread side facing up. Spread mayonnaise on 4 more slices, cut them in half, and place them along the sides of the mold. Put half of the filling in the mold. Place 3 mayonnaise-spread slices on top of that, and then add the remaining filling. Spread mayonnaise on 3 slices and place them on top of the filling.
3. Ensure the top layer of mayonnaise touches the filling. Wrap tightly with the plastic wrap and refrigerate for 3-4 hours, or overnight, to let the flavors meld. Unmold the log and cover the sandwich log with mayonnaise. Garnish with chives and radish.
