1 pound penne pasta
4 tablespoons butter
1 medium yellow onion, chopped
1 tablespoon minced fresh sage
1/2 cup flour
5 cups milk
1 teaspoon salt
1/2 teaspoon ground black pepper
1/8 teaspoon cayenne pepper
2 cups grated Gruyere Cheese
2 cups grated Parmesan Cheese
1 cup grated Fontina Cheese
1 cup crumbled Blue Cheese or Gorgonzola
6 ounces thinly sliced prosciutto, cut crosswise into 1/3-inch strips
1 cup oil-packed sun-dried tomatoes, drained and chopped
Preheat the oven to 375F degrees. Bring a large pot of salted water to a boil. Add the penne and cook until al dente, according to the manufacturer's instructions. Drain the pasta and toss it with a splash of olive oil to prevent it from sticking together.
Butter a large (about 3 quart) casserole dish or individual oven-proof dishes, if using.
In a large saucepan set over medium heat, melt the 4 tablespoons of butter. Add the onion and cook until softened and lightly golden, 5-6 minutes. Add the sage and cook, stirring constantly, for 30 seconds. Add the flour and stir constantly for one minute more.
Slowly add the milk to the saucepan while continuing to stir. Cook the mixture, stirring, until the mixture comes to a boil and thickens. Turn off the heat and stir in the salt, pepper, and cayenne. Add all four cheeses, the sun-dried tomatoes, and the prosciutto to the mixture, stirring until the cheese has melted.
Add the reserved penne to the cheese sauce, stirring until well incorporated. Transfer the mixture to the buttered casserole dish or divide it among the individual dishes set on a baking sheet. Bake until the cheese sauce is bubbling and the top is browned, about 20-30 minutes, depending on the size of the baking dish. Transfer the dish(es) to a wire rack to cool for 10 minutes before serving.