1 medium red bell pepper (about 8 oz), cut into 1/2-inch dice
2 tsp dried oregano
1/4 cup all-purpose flour
1 1/2 cups whole or reduced-fat milk
1/2 cup heavy cream
12 oz cheddar cheese, coarsely grated (3 1/2 cups)
2 tsp hot sauce
1/2 tsp dry mustard powder
Ketchup, for serving (optional)
Position an oven rack about 6 inches below the broiler and preheat to broil. Butter an 8-inch square (1 1/2-quart) metal pan or six 8-ounce ramekins. Set aside. (Note: Do not use a glass pan or a ceramic dish for this recipe. It can break when set under the broiler.)
Fill a 4- to 5-quart pot about three-quarters full with water and add 1 tablespoon of the salt. Bring to a boil and add the pasta. Cook, stirring once or twice, until tender but firm, 4 to 6 minutes for small shells, 8 to 10 minutes for medium shells. Drain, and reserve the pot.
In a large skillet, heat the 1/4 cup of the oil over medium heat. Add the potatoes and cook, stirring occasionally, until the edges are darkened and the potatoes are crisp, 10 to 12 minutes. Add salt and black pepper to taste. Using a slotted spatula or spoon, transfer the potatoes to a plate.
Using the same skillet, cook the onion and bell peppers, stirring occasionally, until the vegetables are soft and beginning to caramelize (darken) around the edges, 8 to 10 minutes. Add the oregano and salt and black pepper to taste. Turn off the heat and set aside.
Using the same pot you used to cook the pasta, heat the remaining 2 tablespoons oil over medium heat. Slowly whisk in the flour and the remaining 1/2 teaspoon salt and stir constantly until a paste forms, 30 to 45 seconds.
Continue stirring for 1 to 2 minutes more, until the mixture starts to darken slightly and smell a bit nutty. Slowly whisk in the milk and cream and stir until the mixture starts to thicken and is just beginning to bubble around the edges, 5 to 7 minutes. It should be thick enough to coat the back of a wooden spoon.
Add 3 cups of the cheese, the hot sauce and mustard powder and stir until the cheese has melted and the sauce is smooth but not too runny. It should be similar in texture to cake batter. If it's soupy, continue cooking until it thickens. Turn off the heat and add the pasta and peppers. Stir to combine. Transfer the mixture to the prepared dish. Pile the potatoes on top of the casserole and sprinkle the remaining cheese over the potatoes.
Put the pan on a rimmed baking sheet and place under the broiler. Cook until the cheese is bubbly and golden brown and the potatoes that are peeking out begin to darken, 2 to 3 minutes. Watch carefully, because the cheese and potatoes can burn easily. Let cool for 10 to 15 minutes before serving. Pass extra hot sauce and/or ketchup, if desired, alongside.