Preheat oven to 350°F (175°C) and lightly grease a 9x5-inch loaf pan or line it with parchment paper.
In a large bowl, combine ground beef, diced onion, eggs, tomato sauce, Worcestershire sauce, quick oats, garlic salt, onion salt, and pepper.
Mix gently with clean hands or a wooden spoon until just combined — do not overmix.
Transfer mixture into the prepared loaf pan, shaping it into an even loaf.
In a small bowl, whisk together ketchup, brown sugar, and mustard until smooth.
Spread half of the glaze over the top of the meatloaf.
Bake uncovered for 45 minutes, then remove from the oven and spread the remaining glaze on top.
Return to the oven and bake for another 10 minutes, or until internal temperature reaches 160°F (71°C).
Let the meatloaf rest for 5–10 minutes before slicing to retain juices.
Slice, serve, and enjoy with mashed potatoes or roasted vegetables.