While potatoes cook, heat oil or butter in a large skillet over medium heat. Add sausage and cook until browned.
Add onion to the skillet and cook 2-3 minutes until softened.
Add cabbage and cook 6-8 minutes, stirring occasionally, until tender.
While cabbage is cooking, prepare the mashed potatoes according to the package directions.
Serve sausage and cabbage over mashed potatoes.