In a Dutch oven, brown pork in melted butter and oil. Remove from pan.
Sauté mushrooms and onion. Season with salt and pepper.
Cook for about 5 minutes on medium high heat, tossing so mushrooms brown a bit.
Reduce heat, add flour and stir until smooth.
Add chicken broth, Worcestershire and fresh thyme. Stir.
Place meat back into pan.
Cover with foil or tight fitting lid and bake at 350 degrees for 1 ½- 2 hours*.
Notes
-I usually turn off the oven after 2 hours cooking time, then let meat sit in the oven, covered until ready to cut and serve. Last Sunday, it was in the oven for about 1 1/2 hours past cooking time, and then on the counter covered for about another hour. It never dries out, and always falls apart when cutting.
-I have cut the amount of butter by half, and the recipe still works.
-Don't cut the amount of mushrooms or onions, unless you substitute another vegetable that yields a high amount of water, or there won't be enough gravy to keep the meat moist.
-Costco sells Pork Tenderloin in a two pack. Each sleeve has two tenderloins, so 4 tenderloin per purchase, enough for a double recipe. Costco also sells mushrooms, whole in a 1 1/2 lb package. I use a mixture of their white and brown mushrooms, but all white works as well.
-I have tried this in the Instant Pot and the cook time is 6 minutes on high pressure, then natural release which takes about 15 minutes. I cook (saute) the onion and mushrooms in a pan on the stove and then add flour and broth to the pan on stove. After the meat is finished being pressure cooked, Remove the meat from the IP, slice and add to the ingredients from the stove top and serve.