Cook onion in butter till tender but not brown.
Prepare dough as directed on hot-roll-mix package, adding onion to the dry mix.
Cover and let rise in a warm place until double (30 to 60 minutes).
Shape in 12 round buns.
Place on greased baking sheet and flatten slightly.
Cover and let rise until almost double (30 to 60 minutes).
Bake in moderate oven (375°) for about 20 to 25 minutes.