Heat the oven to 400°F. Line a rimmed baking sheet with parchment paper.
Warm 1 tablespoon of olive oil in a large skillet over medium heat. Add the sliced onions and cook for 15 to 20 minutes, stirring often, until soft and deeply golden. Stir in the thyme, then set the onions aside to cool slightly.
In a large bowl, combine the ground turkey, ground beef, egg, breadcrumbs, milk, Dijon mustard, Worcestershire sauce, salt, pepper, and ¾ cup of the Gruyère. Add the cooled onions and mix gently until just combined.
Shape the mixture into a compact loaf in the center of the prepared baking sheet. Avoid packing it too tightly so the meatloaf stays tender.
Toss the carrots and potatoes with the remaining tablespoon of olive oil. Arrange them around the meatloaf in a single layer, leaving a little space between the pieces.
Bake for 40 minutes. Sprinkle the remaining ¼ cup of Gruyère over the meatloaf, then bake for another 10 to 15 minutes, until the cheese is melted and the center of the loaf reaches 165°F.
Let the meatloaf rest for 10 minutes before slicing. Serve it with the roasted carrots and potatoes.