In a large skillet over medium-low heat, saute the chicken in olive oil until golden brown. The internal temperature should be 160℉. Remove the chicken to a plate.
Add the mushrooms and onion to the skillet and saute until the mushrooms have started to brown and they have released most of their water. If the bottom of the skillet looks like it might burn, add a few tablespoons of water to deglaze the pan.
Add the garlic and saute for 30 seconds.
Add the butter and allow to melt. Add the flour and combine to make a roux. Cook for about two minutes.
Add the chicken base mixture and the half-and-half. Raise the heat and stir, scraping the bottom of the skillet, until the gravy comes to a near-boil. This will activate the roux and thicken the gravy.
Add worcestershire sauce, dijon mustard, thyme, and red pepper flakes. Simmer for a few minutes.
Add the chicken back to the skillet, and simmer until chicken is warm.
Serve over pappardelle, your favorite pasta, or mashed potatoes.