For the caramelized onions:
1 large sweet onion, cut in half and thinly sliced into half-moons
1 tablespoon olive oil
1 tablespoon butter
Salt, to taste
For the pasta:
8 ounces penne pasta
3 tablespoons butter
3 tablespoons flour
1 1/2 cups heavy cream
1 1/2 cups milk
1 cup shredded parmesan cheese
Pinch of ground nutmeg
Pinch of cayenne pepper
3 cloves garlic, minced
Salt and pepper, to taste
Make the caramelized onions: Heat a large, heavy skillet to medium. Add butter, oil and onions; stir so that all of the onions are coated with the butter and oil. Cook, stirring every few minutes, until the onions soften and turn a golden caramel color. The entire process will take about 30 minutes. Once the onions are caramelized, season lightly with salt, remove from heat and set aside.
Bring a large pot of water to a boil and season liberally with salt. Add pasta and cook for exactly 6 minutes. Drain in a colander and set aside.
Heat the same pot you cooked the pasta in to medium. Add butter and let melt. Add garlic and cook for 1 minute. Add flour and stir so that the flour is coated with the butter. Let cook for 2 minutes. Gradually whisk in the milk, about 1/2 cup at a time; whisk constantly to break up and lumps. When the first addition of milk begins to thicken, add more. Repeat this process until all of the milk and cream have been added. Let mixture continue to cook until it is thick enough to coat the back of a spoon. Once thickened, turn off heat, stir in nutmeg, cayenne and cheese. Taste and season with salt and pepper.
Add drained pasta and onions to the sauce and fold to combine. Pour entire mixture into a greased 9" x 13" baking dish. Bake for 15-20 minutes, until the pasta is golden and bubbling. Let stand for a few minutes before serving.