Heat butter and oil in a large saute pan over medium-high heat.
Add onion and apple slices and saute for 4-5 minutes, until softened and slightly browned.
Meanwhile, season the chicken strips with 2 teaspoons of rosemary, salt and black pepper.
Push the onion and apple slices to the sides of the pan and add chicken breast strips to the middle. Brown on both sides, about 6-8 minutes total. (If your pan is too crowded, do this in batches so you can properly brown the chicken.)
Add vinegar and remaining teaspoon of rosemary then cover the pan, reduce heat to medium and cook until chicken is cooked through, 3-5 more minutes.
Serve chicken and apples with extra pan juices and enjoy!
