Preheat your oven to 450℉ and line a large sheet pan with parchment.
In a mixing bowl, stir together the breadcrumbs, diced onion, garlic, thyme, salt and pepper until well combined.
Add the ground beef, egg, Worcestershire sauce, Dijon mustard, and ¼ cup of the shredded Gruyère cheese and stir to combine (I use my hands) .
Form 20 equal meatballs using a 1.5 inch cookie scoop and place them in a single layer on the prepared sheet pan. Bake in the preheated oven for 14-15 minutes, or until they reach an internal temperature of 160 ℉.
Meanwhile, make the gravy. Heat the butter over medium heat in an oven-proof skillet. Once melted, add the sliced onions and cook until slightly browned and caramelized, about 8-10 minutes. Add the garlic and saute another 30 seconds.
Whisk together the beef broth and cornstarch until the cornstarch is dissolved (make sure the beef broth is cold or room temp, not warm/hot). Pour the mixture into the skillet.
Bring the mixture to a simmer while scraping the brown bits from the bottom of the pan. Continue to cook until the mixture is thickened (about 2-3 minutes)
By now, the meatballs should be cooked and out of the oven. Add the cooked meatballs to the skillet and toss them in the gravy. Sprinkle the remaining shredded Gruyère over the top, and place them in the oven on the broil setting for 2-3 minutes until golden brown. Serve!