Heat oil in saucepan. Add onion and cook until softened. Add beans and salsa. Cook, stirring occasionally for 15 minutes. Add cooked chicken and cook for 5 minutes. Spread 1/3 of bean mixture in baking dish. Cover w/half tortilla strips. Repeat layers and top w/remaining bean mixture. Sprinkle cheese over bean mixture. Bake for 20 minutes at 350 degrees. (My note: I didn't use the medium onion since the salsa had onions in it).