Heat 1 to 2 tablespoons of olive oil in a wide skillet over medium heat. Add the thinly sliced onions and a pinch of salt. Cook, stirring occasionally, until the onions are deeply golden and fragrant, about 20 to 25 minutes. Lower the heat if they start to brown too quickly. Remove from heat and let cool slightly.

In a large bowl, combine the ground beef, breadcrumbs, shredded cheese, egg, garlic powder, thyme, salt, and pepper. If using onion soup mix, add it here.

Add the cooled caramelized onions to the meat mixture. Gently fold with your hands or a spatula until just combined. Avoid overmixing to keep the loaf tender.

Shape the mixture into a compact loaf and place it on a parchment-lined baking sheet or in a lightly greased loaf pan.

Preheat the oven to 375°F (190°C). Bake the meatloaf for 50 to 60 minutes, or until an instant-read thermometer registers 160°F (71°C) for beef and 165°F (74°C) for turkey.

Remove the meatloaf and let it rest for 5 to 10 minutes before slicing. This helps the juices redistribute so slices hold together.