Heat olive oil in skillet over medium-high heat. Salt and pepper the chops; put in skillet, in batches if necessary, and brown quickly on both sides. Remove to a baking dish lightly sprayed with nonstick cooking spray.
To the hot skillet, add chopped onions and mushrooms. Cook, stirring, until onions are browned and just tender. Add chicken broth to the pan.
Stir together Ranch dressing and mustard; add to broth. Let the mixture boil for a minute; pour over chops in baking dish.
Cover baking dish with foil and bake at 325° F. for 1 1/2 hours.