Melt 2 tablespoons of the butter in a large skillet over high heat, and add the onions.
Cook, stirring well to coat onions with the butter for a couple minutes.
Then turn down to medium-low, season lightly with some salt & pepper and add the thyme - mix well and continue cooking, stirring often until onions are beautifully browned, 15 to 20 minutes.
This is the perfect time to boil your pasta - but def don’t overcook - keep them al dente.
Add vermouth to the onions, scraping along the bottom to get up the ‘fond’, let vermouth evaporate, and add garlic.
When fragrant, add the beef broth and cream - bring to a boil, and turn to a low simmer until slightly thickened - about 5 minutes.
Preheat oven to broil.
Add the cooked pasta and most of the cheeses (keep back about ½ cup between the two).
Stir until nicely melted and combined, and put in a serving dish.
Sprinkle with the crispy onions, top with remaining cheese and put under the broiler until just melted.
Serve.
