1. Peel the potatoes and cook in water with a little salt. Drain, mash with a fork, and season with salt and pepper. Add milk and mix. Set aside.
2. Cut the tomatoes in half and remove the seeds. Chop tomatoes and onion, carrot, garlic, and pepper. Sauté in a pan with the oil until well cooked and tender, adding a little water if necessary. Set aside.
3. Chop the boiled egg, add drained tuna, mayonnaise and mix well. Set aside.
4. Purée the lettuce with a tablespoon of water using an immersion blender, and then drain excess liquid. Mix lettuce with mayonnaise.
5. Using a ring mould, place a layer of potato first, then a layer of sauteed vegetables, followed by a layer of the egg and tuna mixture, and then another layer of potato. Top off with a layer of lettuce as a garnish. Remove mould and serve.