Set an oven rack 6 inches from the heat source and preheat the oven to 425 degrees F.
Bring a large pot of salted water to a boil. Add the macaroni and cook until it is al dente, about 6 minutes. Strain, reserving 1 3/4 cups of the pasta water. Set aside.
Heat the half-and-half, oregano and crushed red pepper in a large saucepan over medium heat until the mixture just comes to a simmer. Continue simmering until the half-and-half is reduced to 1 1/2 cups, about 15 minutes. Add the cream cheese and stir until melted. Whisk in the Cheddar and Gruyere until all the cheese is melted and the sauce is smooth.
Add the macaroni and reserved pasta water to the saucepan and stir to combine. (The mixture will look very loose, but the pasta will suck up a lot of the sauce as it bakes.) Gently stir in the marinara sauce, sun-dried tomatoes and pepperoni and season with more salt if needed. Pour into a 13-by-9-inch baking dish. Toss the bread crumbs, Parmesan and sun-dried tomato oil together in a small bowl and sprinkle evenly over the macaroni and cheese. Bake until the cheese is bubbly and the bread crumbs are lightly browned and crispy, about 10 minutes.
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