In a large frying pan, melt butter over medium heat. Add onion slices and cook 20 minutes. Stir onions frequently to keep from burning. Turn off heat and add red bell peppers, chicken and cream cheese. Stir until cream cheese has melted and ingredients are combined. Spread chicken mixture into a 2 quart casserole dish. Pour enchilada sauce over chicken mixture and top with grated cheese. Dip can be refrigerated at this point, if desired. Preheat oven to 375 degrees and place casserole in oven. If refrigerated, bake uncovered for 30 minutes or until cheese is hot and bubbly. If not refrigerated, bake 20 minutes or until cheese is hot and bubbly. Remove from oven and sprinkle with chopped cilantro, if desired. Serve with corn tortilla chips for dipping.