Preheat your oven to 375°F and line a small loaf pan with parchment paper.
Finely mince the mushrooms and onions, then sauté them in a non-stick skillet over medium heat until all moisture has evaporated and they are soft.
In a large mixing bowl, combine the ground beef, sautéed mushroom and onion mixture, minced garlic, egg, coconut aminos, sea salt, black pepper, and dried thyme.
Gently mix the ingredients by hand until just incorporated, taking care not to over-compress the meat.
Transfer the mixture to the prepared loaf pan and shape it into an even loaf.
In a small ramekin, whisk together the tomato paste and balsamic vinegar until smooth, then brush the glaze over the top of the meatloaf.
Bake for 35 to 40 minutes or until the internal temperature reaches 160°F.
Allow the meatloaf to rest for at least 5 minutes before slicing to ensure the juices redistribute for a moist texture.