Spinach Mushroom-Stuffed Beef Tenderloin

Entree

Cook mushrooms in butter in a medium skillet over medium-high heat, stirring constantly, until tender. Remove from heat; stir in spinach, Parmesan cheese, and next 5 ingredients.
Slice tenderloin lengthwise down center, cutting to, but not through, bottom. Spoon spinach mixture into opening of tenderloin. Press sides of tenderloin together over filling, and tie securely with heavy string at 2-inch intervals. Combine 1 teaspoon pepper, ¾ teaspoon fennel seeds, and garlic; rub over entire surface of tenderloin.
Place tenderloin on a lightly greased rack in a shallow roasting pan; insert meat thermometer into thickest portion of tenderloin. Place tenderloin in a 500 degree oven; immediately reduce oven temperature to 350 degree. Bake, uncovered, for 50 to 60 minutes or until meat thermometer registers desired degree of doneness (rare 140 degree, medium-rare 150 degree, or medium 160 degree). Let stand 10 minutes before slicing.