Add butter to a heavy bottomed skillet over medium heat. Once melted, add onions, olive oil, and a hefty pinch of salt. Cook for 20 minutes, stirring often until soft and starting to brown.
Add garlic and thyme and cook for 3 more minutes or until the garlic is tender and fragrant.
Add balsamic vinegar and white wine to deglaze the pan - scraping up all the little bits on the bottom of the pan.
Add rigatoni, Worcestershire sauce, and beef broth and stir everything together. Carefully submerge the noodles and cook over a medium simmer for 20 minutes with the lid on, occasionally removing it and stirring so the noodles don’t stick to the pot. The liquid should absorb into the uncooked pasta - if it's still watery after this step, leave the lid off and simmer for a few minutes or until it thicks back up.
Once al dente, add the @emmicheeseusa Gruyère and heavy cream and gently stir until creamy. Taste and season with flaky salt and pepper.
