Salt and pepper the chicken breasts on both sides.

Heat a very large skillet over medium-high heat with enough olive oil and about 1 tablespoon butter. Put the chicken breasts in the pan and cook for about 5-7 minutes on each side, or until golden brown and cooked through. Remove from the skillet and set aside.

In the same skillet, melt the remaining tablespoon of butter. Add the chopped onion and minced garlic, cooking until the onion is transparent. Add the sliced mushrooms to the mixture and cook until tender.

Pour in the chicken broth, making sure to scrape any caramelized bits off the bottom of the skillet. Stir in the Dijon mustard, heavy cream, and Worcestershire sauce, then let the sauce simmer for about 3-5 minutes until it has thickened a bit.

Return the cooked chicken to the skillet and spoon over the sauce. Continue to cook for 2-3 minutes just to heat through.

Garnish with chopped fresh parsley and serve hot. Chicken Diane pairs beautifully with rice, pasta, or a crisp green salad.