In a Dutch oven, melted butter with oil. Add in tenderloin and brown. Remove from pan and set aside.
In the same dutch oven, sauté mushrooms and onion together. Season with salt and pepper. Cook for about 5 minutes on medium heat until tender. Reduce heat, add flour and stir until smooth.
Add chicken broth, Worcestershire, fresh thyme, and a dash of browning sauce. Stir together until flour is blended in.
Place meat back into Dutch oven. Cover with foil or tight fitting lid and bake at 350 degrees for 1 ½- 2 hours. Slice and serve with hot cooked mashed potatoes or rice. If gravy isn’t thick enough you can make a slurry of flour and water to thicken it more. Serves 8.