Preheat your oven to 375°F and line a small baking dish or loaf pan with parchment paper.
Finely chop the cremini mushrooms, yellow onion, and garlic until they are very small, uniform pieces.
In a large mixing bowl, combine the ground beef, chopped mushrooms, onion, garlic, egg, panko breadcrumbs, coconut aminos, sea salt, and black pepper.
Use your hands to gently mix the ingredients until just combined, being careful not to overwork the meat.
Shape the mixture into a loaf and place it into the prepared baking dish.
In a small bowl, whisk together the tomato paste and yellow mustard, then brush this glaze evenly 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 5 minutes before slicing to ensure it stays juicy.