1. Peel the potatoes and boil in water with a little salt. Drain, mash with a fork, and season with salt and pepper. Pass the potato through a sieve. Add milk and mix thoroughly. Set aside.
2. Chop tomatoes, peel and chop onion, carrot, garlic, and pepper. Gently sauté with the oil until well-cooked. Season. Purée with an immersion blender and pass through sieve. Set aside.
3. Mix hard-boiled egg with drained tuna and 20 g of mayonnaise. Purée together and pass through sieve to obtain a fine purée without any lumps. Set aside.
4. Purée clean lettuce with a tablespoon and a half of water using an immersion blender. Pass through sieve and mix with remaining mayonnaise. Add the thickening agent and continue puréeing.
5. Use a ring mould to place a layer of potato first, then a layer of sautéed 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.
The amount of thickening agent necessary depends on the brand used. It is important that all the purées have the same texture.