Remove the outer leaves from the heads of lettuce. Then wash and dry well. Cut the robiola into cubes and toss with a little oil, lemon juice, a pinch of salt and a pinch of pepper. Wash and dry the tomatoes. Peel and remove the seeds. Cut into fillets and then dice. Stuff the heads of lettuce with the robiola and chopped tomatoes. Wash and dry the tomatoes for the sauce. Peel, squeeze and pass through a food mill. Let all the tomato puree drip out then add the oil, vinegar, salt, pepper and finely chopped basil leaves. Cut the heads of lettuce into four parts. Arrange on a large serving dish and cover with the sauce and serve.