In 1-2 tablespoons oil, saute the cabbage, garlic, and white/yellow onion for a few minutes until the cabbage starts to get tender. Set aside. Boil the potatoes until fork tender. Drain and mash with 1/2-1 stick butter. Salt and pepper to taste. Add enough milk to mash to desired consistency. Into the mashed potatoes, fold in the cabbage/onion mixture. Add in the cheese (reserve a little for top) and the green onions (add a few for sprinkling on top).