French Onion Meatballs 9 Steps
Soak the stale bread cubes in with the whole milk until most of the milk has absorbed, approximately 10 minutes. Squeeze out and discard the excess milk.
In a large mixing bowl, combine the soaked bread cubes, ground beef, diced yellow onion, minced garlic, chopped parsley, whole egg, as well as kosher salt and fresh cracked black pepper, mix well until even. Begin forming meatballs approximately 1 ½" in diameter (you will yield between 15-20 balls) and place on a parchment paper-lined baking sheet. Bake all of the formed meatballs in a preheated 350°F oven for 30-35 minutes, then remove and set aside.
While the meatballs are baking, prepare the French onion base: warm a heavy bottomed skillet on medium high heat and add the 2 tablespoons of olive oil as well as 2 tablespoons of butter, once the butter has melted add in the sliced onions, granulated sugar and 3/4 teaspoon of kosher salt (to start). Continue to cook for 5 minutes stirring often until the onions start to turn a light brown. Turn the heat to medium low, cover and cook for 15-20 minutes stirring regularly until the onions have softened completely and turned a deep caramel colour, add in the minced garlic as well as fresh thyme leaves and continue to cook for another minute.
Deglaze the caramelized onions with the sherry, being sure to stir up any browned bits from the bottom of the pan. Add in the beef stock and simmer for 5-7 minutes.
While the French onion base is simmering, dust the cooked meatballs with the all-purpose flour and shake off any excess. Warm a skillet on medium high heat, add in the 1 tablespoon of olive oil as well as the 1 tablespoon of the butter and begin searing the meatballs until they are all browned on most sides.
Add the seared meatballs to the simmering French onion base and cook on high heat for about 5 minutes until the liquid has reduced by about half and the sauce is glaze-like. Add in the red wine vinegar and adjust seasoning with kosher salt and fresh cracked black pepper.
To finish: brush olive oil on the sourdough slices and season lightly with kosher salt and fresh cracked black pepper and toast in a preheated 400°F oven until golden brown.
Transfer the French onion meatball mix to an appropriate sized baking dish and top with the grated Gruyere, grated mozzarella as well as grated Parmigiano-Reggiano and broil on high for 5-7 minutes until the cheese is bubbly and golden brown. Remove from the oven and sprinkle with additional thyme leaves if desired. Serve immediately with toasted bread slices.
Note: an alternative way to serve would be to top each toasted slice of bread with the meatballs and onion mix, then the cheeses and then broil.