2 large portobello mushrooms, stemmed, black gills cut off, caps sliced 1/4
1 medium red onion, thinly sliced
2 garlic cloves, thinly sliced
1/4 cup white wine vinegar
1/2 cup chicken stock or low-sodium broth
1 teaspoon chopped thyme
In a large skillet, heat the olive oil. Season the chicken breasts with salt and pepper and add them to the skillet. Scatter the mushrooms, onion slices and garlic around the chicken and cook over high heat until the chicken is browned on the bottom, about 3 minutes. Turn the breasts over, stir the mushrooms, onions and garlic and cook until the chicken is browned on the second side, about 3 minutes longer.
Add the vinegar, chicken stock and thyme to the skillet and simmer, scraping up the browned bits from the bottom of the pan. Reduce the heat to low and cook, turning the chicken once, until white throughout, about 7 minutes. Transfer the chicken to warmed plates. Simmer the sauce over moderate heat until slightly reduced and flavorful, about 1 minute. Season the sauce with salt and pepper, pour it over the chicken breasts and serve.