1. Heat oil in large skillet over medium-high heat. Brown chicken pieces and cook until heated through.
2. Add taco seasoning and water (amount specified on packet) to chicken, bring to a boil, then simmer for 5-10 minutes. Remove chicken and set aside.
3. Reduce stovetop heat to medium. Butter one side of a tortilla and place it, butter side down, in the skillet. Add 1/2 cup cheese, 1/4 of the onions, and some chicken strips to the top of the tortilla.
4. Butter another tortilla and place it, butter side up, on top of the chicken and cheese until bottom tortilla is golden brown (3-5 minutes). Flip the quesadilla with a large spatula and cook until the other side is golden brown.
5. Repeat with remaining ingredients. Serve with salsa and sour cream.