3 tbsp olive oil
3 lg red and/or yellow peppers — cut in strips
2 c very thinly sliced onions
1 tsp minced garlic — to taste
3/4 tsp each salt and pepper
4 tsp red wine vinegar
1/2 tsp sugar
8 skinned, boned chicken thighs — visible fat removed
1 tbsp butter or margarine
Heat 2 tablespoons of the oil in a large, heavy nonstick skillet over medium heat. Add peppers and onions and saute until soft and onions are slightly browned, about 20 minutes. Removed from heat, stir in the garlic, 1/4 tsp EACH of the salt and pepper, the vinegar and sugar. Scrape into a bowl. Wipe pan clean with a paper towel.
Season chicken with remaining salt and pepper. Heat remaining 1 tablespoon oil and the butter in the skillet until bubbly. Add thighs ad cook, turning occasionally, until opaque in center and brown on both sides, about 20 minutes. Remove from pan and pour off fat.
Return chicken and bell pepper mixture to skillet and cook stir until hot, about 3 minutes.