In a large skillet, melt the butter or bacon grease over low heat. Add your thinly sliced onions and a pinch of salt, tossing them gently to coat. Cook patiently, stirring often, until the onions are beautifully soft and golden brown – this takes about 15 to 20 minutes.
Stir in the fresh thyme leaves and cook for just one more minute to release their aroma. Then, take the skillet off the heat and set the onions aside.
Preheat your oven to 350°F (175°C) and line a large baking sheet with parchment paper. In a big bowl, combine the ground beef and pork, ground pork rinds (or almond flour), onion powder, black pepper, and 1/2 teaspoon of salt. Crack in the egg and add half of your lovely caramelized onions. Use your hands to thoroughly mix everything together until well combined.
Transfer the meat mixture onto your prepared baking sheet and gently shape it into a neat loaf, roughly 5 by 9 inches. Bake in the preheated oven for 40 to 50 minutes, or until the internal temperature reaches 160°F (71°C). Once cooked, remove the meatloaf from the oven and turn the oven off.
Sprinkle the grated Gruyere cheese over the top of the hot meatloaf, along with the remaining caramelized onions. Pop it back into the warm, turned-off oven for about 5 minutes, just until the cheese is melted and bubbly.
For the best slices, let your French Onion Meatloaf rest for 10 to 15 minutes before serving. This helps keep it juicy and easy to cut!