Warm a large oven-safe skillet over medium-low heat and pour in 2 tablespoons of olive oil. Add the sliced onions along with a generous pinch of salt. Cook the onions slowly, stirring frequently, until they are deeply caramelized and golden brown, about 30 to 40 minutes. Transfer them to a plate once done.

Preheat the oven to 350°F. Roughly chop half of the caramelized onions for the meatball mixture.

In a mixing bowl, combine the ground meat with the minced garlic, ½ cup of gruyere, all of the parmesan, breadcrumbs, parsley, thyme, salt, and pepper. Fold in the chopped onions. Gently mix until just combined to maintain tenderness.

Shape the mixture into 1-inch meatballs using moistened hands for ease. Uniformity isn’t essential—slightly rustic shapes are fine.

Reheat the same skillet over medium heat, adding a light coating of olive oil. Arrange the meatballs in a single layer and sear them, turning every 1 to 2 minutes, until they are browned on all sides.

In a shaker cup or jar, vigorously mix the stock with the flour for 30 seconds until well combined. Pour this mixture into the skillet over the meatballs. Stir in the remaining caramelized onions and scatter the remaining gruyere over the top.

Place the skillet in the oven and bake for 20 to 25 minutes, or until the internal temperature of the meatballs reaches 165°F. Serve hot, ideally over toasted sourdough for a comforting twist on the meatball melt.