Caramelized Onions

1. In a large skillet over low- medium heat add the butter and onions. Stir every five minutes or so, it will take about 35-45 minutes for the onions to caramelize depending on how large your pan is. While the onions are cooking make the meatballs.

Meatballs

1. In a large bowl mix together the ground beef, bread crumbs, thyme, garlic, egg, milk, Worcestershire sauce, salt & pepper, and shredded gruyere cheese. I use my clean hands to mix until just combined, don't over-mix it. I use a medium cookie scoop to scoop equal size meatballs then use my hands to roll them into balls. Place on a plate and lightly cover with plastic wrap then set aside until ready.
2. Once the onions are a deep brown, sprinkle some flour on top and mix to combine for a minute or two. Add the white wine and the back of your wooden spoon to scrape the bottom of the pan, all those brown bits are the flavor.  Add the beef stock and balsamic vinegar and cook for another 3-4 minutes. Once thickened transfer to a heat-proof bowl.
3. In the same pan (it's okay if there are a couple of onions left) brown the meatballs over medium heat. Turn the meatballs every 2-3 minutes to cook on each side.  Add the onions and sauce back to the pan, and push it around so that the meatballs sort of sit on the onion sauce. Sprinkle on cheese and broil for 3-5 minutes, watch carefully so you don't burn the cheese. The meatballs should be fully cooked by now but check they have reached 165F/74C.
4. Slice the baguette into bite-size pieces at an angle and brush with olive oil then bake at 425F for 5-10 minutes. Once the crostinis are slightly toasted place each piece around and into the French onion meatballs dish. Store any leftovers in an airtight container for up to 4 days.