Melt butter in a large skillet over medium heat; add onions and cook 20 minutes or until caramelized. Reduce heat to low and add chopped chicken, roasted red bell pepper, cream cheese, salt and pepper. Stir until combined. Reduce heat to simmer.
Pulse diced green chiles, garlic, oregano, cumin, and sugar in a blender or food processor until combined. Pour this mixture into a saucepan. Add chicken broth and boil over high heat. Cook 5 minutes or until slightly thickened. Remove from heat. Spread 1/3 of this chile mixture evenly on the bottom of a lightly greased 13x9 inch baking dish.
Spoon cream cheese chicken mixture down the center of each tortilla; roll up and place, seam side down on chile mixture in casserole dish. Repeat with all tortillas and chicken mixture.
Top with remaining chile mixture; sprinkle with cheese.
Bake at 375 degrees 20 to 25 minutes or until bubbly and cheese is melted.