Melt butter in a large skillet over medium heat; add sliced onions. Cook, stirring often, 30 to 40 minutes, or until onions are caramel colored.
Combine onions, cream cheese, and next 3 ingredients, stirring well. Spoon dip into a lightly greased 1 1/2- to 2-quart casserole dish. Bake, uncovered, at 375° for 30 minutes, or until golden and bubbly. Serve with sweet potato chips. Makes 4 cups.
Dip can be prepared a day ahead, but do not bake. Cover and refrigerate overnight. Bake, uncovered, at 375° for 45 minutes, or until golden and bubbly.