Preheat oven to 375F. Line a large baking dish with parchment paper.
In a large skillet over medium high heat, add the butter to melt.
Add the thinly sliced onions, cover with the lid and cook for 10 minutes. Stir occasionally to keep from burning.
Stir the onions around with a wooden spoon, add ¼ cup water or white wine, then cover to cook another 8-10 minutes. Repeat once more until the onions are deeply caramelized.
Stir in the chopped thyme and sage, as well as the sliced mushrooms in the last 10minutes.
To make the meatballs, you’ll run that slice of artisan bread quickly under running water, then squeeze out as much moisture as you can. Chop it into fine pieces.
In a large bowl, combine the ground beef, minced garlic, beaten egg, chopped bread, and ½ teaspoon of both salt and black pepper. Mix just until combined.
Form the mixture into 2 tbsp sized meatballs. Place on the prepared baking dish.
Drizzle the meatballs generously in olive oil.
Bake 8-10 minutes, just until browned on top.
Stir the chopped thyme and sage into the caramelized onions, then add and distribute the meatballs in the skillet.
Add the beef broth, bring to a boil, then simmer 8-10 minutes.
Top with the baguette slices and shredded gruyere.
Broil on HIGH for 1-3 minutes, until cheese is melted and slightly crisp.
Enjoy warm with Klarbrunn Lime Sparkling Water for a complimentary refreshment!