Bring 2 1/2 quarts of water to a boil in a pot. Add the penne, bring the water back to a boil, and boil the pasta over medium heat, uncovered, for about 8 minutes. (The pasta should be al dente, firm to the bite.) Drain the penne in a colander and rinse it under cold tap water until cool, and set aside.
Heat the butter and oil in a saucepan, and add the flour. Cook the mixture over medium heat for about 10 seconds, then add the milk, stir it in quickly with a whisk so the mixture doesn't scorch. Bring the mixture to a boil, and boil it for 10 seconds. Add the cheddar cheese, salt and pepper, mix well, then cook over low heat for 3 to 4 minutes. Set aside.
When ready to finish the gratin, preheat the oven to 400 degrees.
Mix the pasta with the cheddar sauce, and transfer the mixture to a 6-cup gratin dish. Sprinkle the tomato on top of the pasta and sauce. Combine the Parmesan and paprika in a small bowl, and sprinkle the mixture on the pasta. Bake in a 400 degree oven for 30 minutes, until the gratin is bubbly and nicely browned on top. Serve immediately.
Note: If making the gratin at the last moment, and assembling it while the sauce and pasta are both hot, do not bake the gratin. Instead, place it under a hot broiler until golden brown on top.
Recipe Courtesy of Jacques Pepin