When ready to cook, combine the tomatoes, clams and 1 1/2 cups water in a large saucepan, on the grate, directly over the fire and bring to a boil. Add the pasta to the boiling sauce and stir to coat. Bring back to a boil for 30 seconds then remove from the heat and cover with a lid, let sit for 3 minutes. Uncover and stir in the butter (which will have thawed in the cooler) and the juice of the zested lemon.