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!
Notes
Apples: I use either a regular red McIntosh or a Gala apple in this recipe -- both work great.Butter: You can add an extra tablespoon of butter to this dish at the end, if you want an extra rich sauce.Serving: Serve with potatoes, polenta or rice. Or just some bread.