Bring a large pot of salted water to a boil. Add the macaroni and cook according to package directions for al dente. Reserve 1 3/4 cups of the pasta cooking water and then drain the pasta; set aside.
Heat the oil in a large Dutch oven or saucepot over medium-high heat. Add the sausage and cook until lightly browned, breaking up the meat into small pieces, about 5 minutes. Transfer the browned meat to a plate with a slotted spoon. Add the bell pepper, onion and a pinch of salt to the drippings in the pot and cook, stirring occasionally, until softened and lightly browned, 10 to 12 minutes; transfer to the plate with the sausage.
Carefully wipe out the pot, add the half-and-half and cayenne and bring to a simmer over medium heat. Simmer until reduced to 1 1/2 cups, about 15 minutes. Add the cream cheese and stir until melted. Gradually whisk in the Cheddar and Gruyere, 1 handful at time, until all the cheese is melted and the sauce is smooth. Remove from the heat.
Add the cooked pasta and reserved pasta cooking water to the cheese sauce and stir to combine. Stir in the sausage and vegetables; adjust the seasoning with additional salt. Pour into a 9-by-13-inch baking dish and bake until bubbly and lightly browned, about 10 minutes.
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