In a bowl, mix the tuna with oil from the can, lemon juice, olive oil, vinegar and walnuts. Stir and leave in a cool area to flavor for 10 minutes.
Chop the onion and mash it with salt in a large bowl. Add chopped tomatoes. Clean the lettuce and then rip it by hand - do not cut it with a knife. Mix the green salad with onion and add the flavored tuna.
Toss the salad again and serve it.