In heavy duty freezer Ziploc bag, mix soy sauce, hoisin sauce, oil, garlic, sugar and red pepper flakes. Add tenderloins and stir to coat all over. Seal and refrigerate at least 3 hours, turning bag over occasionally. (I refrigerated this overnight.)
Preheat oven to 500°F. Line small roasting pan with foil. Spray a cooling or cooking rack with nonstick spray, and place on top of pan. Place tenderloins on rack. Add ½ cup water to the marinade bag and pour the liquid into a small saucepan.
Roast meat for 9 minutes. Turn pork and continue to roast on the other side until a thermometer inserted in the center registers 160°F, about 9 minutes more. Remove from oven, and allow meat to rest 8 minutes.
While the meat rests, bring marinade to a low boil. Boil slowly for 6 minutes. Remove from heat and stir butter into the marinade.
Slice tenderloin into ¼ inch slices and arrange over rice. Spoon heated marinade over meat and rice. Top with green onions. Serve immediately.