Spray a 9x13 inch baking dish with nonstick spray.
Heat the butter in a large skillet over medium-high heat. Once melted, stir in the onions with a big pinch of salt and pepper and the thyme. Cook for 10 to 12 minutes, stirring often, until the onions begin to soften. Turn the heat down to low and cook, stirring often, until the onions are dark brown and caramely, anywhere from 1 to 2 hours, or until the onions reach the color you desire. I usually stop around 1 hour since we’re going to cook the onions more in the oven with the meatloaf!
Preheat the oven to 400 degrees F.
To make the meatloaf, combine the chicken, garlic, eggs, breadcrumbs, parmesan, ½ cup gruyere, milk, thyme, oregano and a big pinch of salt and pepper. Mix until just combined. Add in just a quarter of the caramelized onions and mix until combined.
Form the chicken into 4 meatloaves of equal size and place them in the baking dish. Bake for 20 to 25 minutes.
Remove the dish and top the meatloaves with the remaining caramelized onions and the gruyere cheese. Return the pan to the oven and cook for 15 to 20 more minutes, or until the internal temperature reaches 165 degrees F.
Remove from the oven and let cool slightly before serving. Top with fresh chives or thyme.