1Combine flour, salt and pepper in medium bowl. Dredge chicken in flour mixture.
Melt 2 tablespoons of butter in large skillet over medium heat. Sauté chicken until cooked through, about 10-12 minutes per side. Set aside and cover to keep warm.
2Add remaining tablespoon of butter to pan (do not use a new pan or wash it, there is a lot of taste already in the pan, you don't want to wash it away!!). Sauté onions in butter until just starting to brown, about 2 minutes, then add mushrooms. Continue cooking until mushrooms soften and reduce in size, about 5 minutes. Add wine and reduce to simmer.
3Mix corn starch in chicken broth and add to pan. Season with thyme, salt and pepper, adjusting to taste. Let simmer until gravy thickens. Spoon over chicken.