Preheat the oven to 375°F. Line a baking sheet with foil.
In a large skillet, heat the olive oil on medium heat. Add the onions, and cook sauté, stirring occasionally, until softened and slightly golden.
Add the sliced mushrooms and chopped or minced garlic. Cook, stirring occasionally, for about 8 – 10 minutes until the liquid has evaporated and mushrooms and onions are softened and browned.
In a large mixing bowl, add the ground beef, oats, egg, milk, Italian seasoning, salt, pepper, and cooked onions and mushrooms.
Use your hands to mix all the ingredients together until combined. Shape the mixture into a loaf and place on the prepared baking sheet.
Bake uncovered for about 60 - 65 minutes until the internal temperature is 160° F and the meatloaf is cooked through. Rest the meatloaf 10 minutes before slicing.
Optional:
In a small bowl, mix 4 tablespoon tomato sauce and 2 tablespoon ketchup. Spread the sauce over meatloaf then slice and enjoy!