In a large nonstick skillet, saute mushrooms and onions in 1 tablespoon oil until mushrooms are tender. Add half of the garlic; cook 1 minute longer. Add spinach, 1/2 teaspoon salt and cayenne. Cook until the spinach is wilted. Remove from the heat; set aside.
Cut a lengthwise slit down the center of tenderloin to within 3/4 in. of bottom. Open so meat lies flat. Spread with spinach stuffing. Fold one side of meat over stuffing; tie several times with kitchen string. Rub remaining oil over beef. Combine the onion powder, pepper and remaining garlic and salt; rub over beef. Place on a rack in a shallow roasting pan.
Bake, uncovered, at 425° for 40-55 minutes or until a thermometer reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°). Let stand for 10 minutes. Remove string before slicing.