In a large bowl, combine mayonnaise, onion, Parmesan cheese and garlic salt; stir in Swiss cheese. Spoon into quart-sized baking dish.
Bake, uncovered, at 325°F until golden brown, about 40 minutes.
Split apart the Party Potato Rolls and slice rolls in half lengthwise. Gently press the center of each roll half to form mini “scoops,” then arrange on a baking sheet and bake in oven for 2-3 minutes or until lightly toasted.
Remove Cheesy Onion Dip from oven and allow to cool for 1-2 minutes. If desired, sprinkle with parsley. Serve with toasted Martin’s Party Potato Rolls.
Recipe adapted from www.tasteofhome.com