In a large saucepan, saute onion and garlic in oil until tender. Stir in chili powder, salsa, and cumin. Stir in shredded chicken and let it cool.
Grease rimmed 15x10x1 baking pan. Working with one tortilla at a time, spoon a heaping tablespoon of beans down the center of each tortilla. Top with 1/2 cup of the chicken mixture. Fold up the bottom, top and sides of the tortilla; secure with wooden toothpick in necessary.
Place chimichangas in greased baking pan, seam side down. Brush all sides with the olive oil. Bake 20-25 minutes or until golden brown and crisp, turning every 5 minutes. Serve with sour cream and guacamole.