Chop potatoes, boil in salted water 15–20 min until tender.
While potatoes cook, sauté onion in a skillet until soft. Add ground meat, cook until browned.
Stir in ketchup and mustard powder. Add 2–3 Tbsp water to loosen the sauce, simmer 5 min. Taste and adjust seasoning.
Drain potatoes, mash with milk + butter, season with salt/pepper.
Serve sloppy joe mix on buns with mashed potatoes on the side.