Rinse the chicken thighs well. I buy them skinless for lower cholesterol, but you can leave the skin on if you like it crispy. Mix the mustard with the olive oil, oregano, and pepper. Coat the chicken thighs with the mixture and let them marinate for at least 30 minutes to absorb the flavors.

Place the chicken thighs on a baking sheet, bone side up. Bake at 350°F for 20 minutes, then flip and bake for another 20 minutes until golden. If you leave the skin on, use a rack so the fat can drain.

While the chicken is baking, prepare the side. Slice the onion into thin rings, cut the bell peppers into strips and then in half, and slice the mushrooms.

Heat the olive oil and sauté the onion. Once it's soft, add the mushrooms and cook until they release their juices and the liquid evaporates. Finally, add the bell peppers and cook until just tender but still crisp. Season with salt, pepper, and balsamic vinegar.