Preheat oven to 350° Fahrenheit. Line large baking sheet with aluminum foil and set aside.
Add 2 pounds lean ground beef, ½ cup diced white onion, 2 large eggs, 1 cup gluten-free breadcrumbs, 2 cloves garlic, ¼ cup milk of choice, 2 teaspoons salt, and ½ teaspoon freshly ground black pepper to large mixing bowl. Mix all ingredients together with hands until fully incorporated.
Transfer mixture to prepared baking sheet. Form mixture into loaf shape, approximately 8 inches long and 4 inches wide. Set baking sheet aside.
Add ½ cup ketchup, 2 packed tablespoons light brown sugar, and 1 teaspoon dijon mustard to small mixing bowl. Whisk ingredients together vigorously until fully combined.
Brush or pour glaze over top and sides of meatloaf, covering loaf evenly.
Place baking sheet in preheated oven. Bake meatloaf 1 hour, then begin checking doneness with internal meat thermometer. Continue baking meatloaf as needed, checking doneness every 5 to 10 minutes, until meat thermometer reads 155° Fahrenheit when inserted into center of meatloaf.
When target temperature is reached, carefully remove baking sheet from oven and set aside. Let meatloaf rest on baking sheet 5 minutes, then cut meatloaf into 8 equally-sized slices and serve with desired sides.