Cover potatoes with water in a large pot and bring to a boil. Cook until tender, about 10 minutes. Drain and add butter and milk while mashing. Set aside.
Heat oil in pan; add onions and garlic and ground veggie, cook for 5 minutes. Set aside. Drain liquid and add all other ingredients. Mix gently.
Spread ground veggie mixture on the bottom of a baking dish. Spread a layer of mashed potatoes on top.