Chicken in Balsamic glaze

1-2 chicken breasts, or deboned thigh meat (I like dark meat) seasoned with pepper and salt
1-2 tbs olive oil

1 medium-sized onion, thinly sliced
2 cloves garlic, roughly chopped
1/3 cup balsamic vinegar
1 tsp brown sugar (opt)
salt to taste
pepper
1/2 tsp dried whole rosemary

Preheat oven or toaster oven to 350F. Heat olive oil in non-stick pan on high. Sear chicken, both sides, until browned. Place on small baking tray and put in oven for 15 minutes. In same non-stick pan, add onion and saute until browned well or caramelized. Add garlic and rosemary. Stir until incorporated. Add balsamic vinegar and brown sugar. Turn heat to low. Cook until balsamic reduced to about 1/2 its normal volume. Salt and pepper to taste. Serve sauce atop baked chicken.

Serve chicken warm with spring potatoes and side of salad (or some vegetable).