Combine first 7 ingredients in a large plastic or paper bag and shake well. Rinse chicken and add to bag to dredge in flour mixture completely. Shake off excess flour and set aside to set flour on chicken.
In a skillet, melt the butter over medium heat. Saute onions for about 3 minutes. Add garlic and cook for about one more minute being careful not to burn garlic. Remove and set aside. Add olive oil and bring to heat. Add chicken breasts and brown on each side. Set aside.
Add mushrooms and saute for about 3 minutes. Add chicken broth, white wine, chicken boullion cube, dried thyme and stone ground mustard. Wisk to combine and heat over medium heat until reduced by half. Add chicken and onions back to pan and simmer covered for about 20-30 minutes til chicken is cooked through. Sprinkle with chopped parsley and serve immediately.
Serve with fresh tossed salad, steamed asparagus with hollandaise sauce and twice baked potatoes for a truly romantic meal.