Drain the artichokes on a sieve. Coarsely chop the walnuts. Wash sage, dab dry and cut into fine strips. Mix ricotta, walnuts, sage and oil and season with salt and pepper. Clean, wash, quarter, seed and cut the tomato into small cubes. Fill the artichoke bottoms with the ricotta mass and sprinkle with the diced tomatoes. Arrange on a plate as desired on lemon slices and serve garnished with parsley and sage if desired