Slice your onions really thin, about quarter inch slices. Heat a large deep skillet or Dutch oven over medium heat with butter and olive oil. Add all the sliced onions and a big pinch of salt. Stir them occasionally, every few minutes, and let them slowly caramelize for 30 to 40 minutes until they're deep golden brown and sweet smelling. Don't rush this by turning up the heat or they'll burn instead of caramelize. If they start sticking, add a splash of water or broth to deglaze. This is the most important step for French Onion Soup Pasta so don't skip it or rush it.

Once your onions are beautifully caramelized and sweet, add minced garlic and fresh thyme leaves and cook for another 1 to 2 minutes until fragrant. If you're using white wine, add it now to deglaze the pan and scrape up all those brown bits from the bottom with a wooden spoon. Let the wine cook down for 3 to 4 minutes until most of the alcohol smell is gone and the liquid has reduced by about half.

Add your beef broth directly to the pan with the caramelized onions and bring it to a boil over high heat. Once boiling, add your uncooked pasta and stir it around to make sure it's submerged. Reduce heat to medium and let it cook according to package directions (usually 10 to 12 minutes), stirring occasionally so nothing sticks to the bottom. If the liquid cooks off before the pasta is done, add a little more broth or water, about half a cup at a time.

When the pasta is just about al dente and most of the broth is absorbed, turn the heat to low and stir in heavy cream and about 1 cup of shredded Gruyère cheese (save the other cup for topping). Stir continuously until the cheese melts into the sauce and everything becomes creamy and rich. Add salt and black pepper to taste, keeping in mind the cheese and broth are already salty. Let it simmer gently for 2 to 3 minutes until the sauce thickens up and coats the pasta beautifully.

If your skillet is oven-safe, keep the pasta in it. If not, transfer everything to a greased 9x13 inch baking dish. Sprinkle the remaining 1 cup of Gruyère cheese and the grated Parmesan on top in a generous, even layer. Turn your oven broiler to high and position the rack about 6 inches from the heat source. Place the dish under the broiler for 3 to 5 minutes, watching it closely so it doesn't burn. You want the cheese melted and bubbly with golden brown spots on top, just like French onion soup. Remove from oven and let it sit for 5 minutes before serving so the cheese sets slightly and nobody burns their mouth. Garnish with fresh parsley if desired.