Chop the onion and pancetta, saute until lightly browned than mix well with the tuna and ricotta. Boil the pasta until cooked through and cool under cold water, use a piping bag to fill the pasta shells. On low heat melt the butter mix in the flour than pour in the warm milk whisking continuously, finish with the cheese and seasoning. Chop the shallot and garlic, sauté on low heat than simmer in the tomato pulp until sauce in thickened. In a baking dish, place the tomato pulp as a base than the pasta, cover with the cheese sauce, sprinkle some cheese and bake until browned.