Heat olive oil in a skillet over medium heat. Add onions and a pinch of salt. Cook, stirring occasionally, for 10–12 minutes until soft and golden. Stir in balsamic vinegar and Worcestershire sauce. Remove from heat and set aside.
Preheat the oven to 400°F. Line a large sheet pan with parchment paper or foil.
Toss pre-boiled potatoes and Brussels sprouts with olive oil, garlic, salt, and pepper. Spread on one side of the sheet pan. Roast for 15 minutes.
In a large bowl, combine ground beef, ground turkey, egg, mustard, panko, thyme, half of the caramelized onions, salt, and pepper. Mix gently until just combined. Shape into 4 mini loaves.
Remove the pan from the oven and place the meatloaves on the empty side of the sheet pan. Return to the oven and roast for 20–25 minutes.
Spoon remaining caramelized onions over each meatloaf and sprinkle with Gruyère cheese. Return to the oven and bake for 10–12 minutes, until the cheese is melted and bubbly and the meatloaves reach an internal temperature of 165°F.
Spoon pan juices over the meatloaves and vegetables before serving.
