Melt butter in a large skillet over medium-high heat; add onion, and sauté 10 minutes or until tender.
Stir together shredded Swiss cheese and next 5 ingredients; stir in onion, blending well. Spoon mixture into a lightly greased 2-quart baking dish.
Bake at 375° for 25 minutes, and let stand 10 minutes. Serve with tortilla chips or crackers.
To lighten: Substitute vegetable cooking spray for butter; substitute reduced-fat Swiss cheese and light mayonnaise.