How to Make Betty's filet mignon with horseradish brown gravy

Try Betty's tasty steak dish with horseradish brown gravy. This looks absolutely amazing. Play special attention to her description of the cuts and the ingredients.

Ingredients:

.87-oz. package brown gravy mix
2 tablespoons creamy prepared horseradish
(4) 6-oz. beef tenderloin fillets (These may come wrapped with a slice of bacon. I removed the bacon, in order to use less fat. You may leave it or remove it.)
salt, to taste
freshly ground black pepper, to taste
2 tablespoons butter
8-oz. fresh mushrooms, sliced
1 tablespoon peanut oil
cooking oil spray

Prepare brown gravy according to package instructions. (For my package, I whisked 1 cup water into the package mixture, and boiled it over low heat for 1 minute.) Stir 2 tablespoons creamy prepared horseradish into the gravy, and set aside. In a large, deep skillet, place 1 tablespoon peanut oil, enough to cover the bottom. Heat peanut oil over medium heat. When hot, add fillets, and cook 1 minute on each side. (Fillets will be rare.) Place browned fillets in a small (1-quart) casserole dish that has been spray with cooking oil. Sprinkle fillets with salt, and grind fresh pepper over the top, to taste. Pour excess oil from skillet. Add 2 tablespoons butter, and heat over medium heat to melt. Add 8-oz. fresh sliced mushrooms, and cook, stirring constantly, 5 minutes. Remove from heat, and combine with brown horseradish gravy mixture. Pour brown horseradish gravy over fillets. Bake, uncovered, at 350 degrees for 15 to 20 minutes, or until desired degree of doneness. The fillets are medium after about 20 minutes, so you will need to adjust the time if you like them rare or well done.