Boil potatoes in salted water for 10 min. Add broccoli and continue to cook until tender, 2-5 min. Drain and return to pan. Add 3 T butter.
Cook onion in a skillet with oil and remaining butter until soft, 3-4 min. Stir into potato mixture along with Chobani, ricotta, dill, red pepper and 2 t salt.
Transfer to baking dish and bake at 375°F for 10 min. Sprinkle cheddar cheese over top, return to oven and cook until melted, 5-10 min.