Preheat oven to 400 degrees F. Butter or grease a rimmed baking sheet or casserole dish.
In a large bowl, add the grated onion, quick oats (1 cup), milk (1/2 cup), 2 eggs, kosher salt (1 teaspoon), black pepper (1/2 teaspoon), and soy sauce (2 teaspoons) and mix until well combined. Allow to sit for 3-5 minutes while the oats absorb some of the moisture.
Add the ground beef (2 lbs.) to the bowl. Mix well- for best results, use hands.
Divide the mixture into eight parts and form them into loaf shapes. (I like to flatten the mixture in the bowl and use a knife to portion it out into 8 "wedges," kind of like slicing a cake, to get even amounts).
At this point, you can wrap the mini meatloaves in plastic wrap or flash freeze to store in the freezer, or refrigerate until you're ready to bake (see notes for more info on this). Otherwise, proceed to cooking.
Place the mini meatloaves on the prepared baking sheet or casserole dish (as many as you want to cook - if frozen, defrost completely before cooking). Bake at 400 degrees F for 25 minutes (or until the internal temperature is 160 degrees).
This recipes yields 8 mini meatloaves. For a smaller batch, the recipe is easily cut in half. To feed a crowd, it's easily doubled.
For a larger portion/serving size, make 6 meatloaves from the mixture instead of 8 and bake for a few minutes longer.
Make-ahead / freezer directions: After all the loaves have been formed, wrap the ones you don't want to cook right away like burritos in plastic wrap. They can be kept in the fridge for a few days until you are ready to cook, or freezer for up to 6 months. Before cooking, make sure you defrost in the fridge for at least 24 hours to fully unfreeze the meat, otherwise you risk overcooking the outside and undercooking the inside.
Ingredient substitutions: Old-fashioned oats can be used instead of quick (let them sit for a couple minutes longer in the milk/egg mixture). 3/4 cup breadcrumbs may be used instead of oats. Lean ground beef, ground turkey, or even ground lamb, or a mix of ground beef/sausage can be used (they will be a little less juicy with leaner meats). Milk can be omitted or replaced with a plant-based milk for a dairy-free version. 1/2 teaspoon more salt can be used in place of soy sauce.