Brown the hamburger with the diced onion. Drain fat. Once done, make the mashed potatoes and add the extra 2 ounces of butter to the mashed potatoes.
Mix together the browned hamburger, onions, gravy mixes, garlic and onion powder, frozen veggies and the cream of mushroom soup.
Spray the slow cooker with cooking spray.
Pour the mixture into the bottom of the slow cooker and smooth around.
Spread the mashed potatoes on top of the hamburger. Cover and cook on high for 3-4 hours or 6-7 on low. 30 minutes before serving lift the lid (careful to not let it drip into the mashed potatoes. Sprinkle the shredded cheddar cheese over the mashed potatoes, and recover the lid.