Preheat your oven to 375°F and line a small loaf pan with parchment paper for easy removal.
Finely mince the yellow onion and garlic to ensure they blend seamlessly into the meat mixture.
In a large mixing bowl, combine the lean ground beef, egg, oat flour, minced onion, and garlic.
Add the tomato paste, yellow mustard, sea salt, and black pepper to the bowl.
Gently mix the ingredients with your hands until just combined, being careful not to overwork the meat which can make it tough.
Transfer the mixture to the prepared loaf pan and press it down into an even, flat-topped loaf shape.
In a small ramekin, whisk together the unsweetened ketchup and apple cider vinegar to create the glaze.
Spread the glaze evenly over the top of the meatloaf using the back of a spoon.
Bake for 35 to 40 minutes or until the internal temperature reaches 160°F and the glaze is bubbly.
Remove from the oven and let the meatloaf rest for 5 to 10 minutes before slicing to lock in the juices.