Trim excess fat and silverskin from tenderloin. Cut into 1-inch pieces, pat with paper towels, rub with some sesame oil, and season with salt and freshly ground black pepper.
Place a medium skillet over medium high heat, add 1 tablespoon sesame oil. When oil shimmers, add pork and cook until browned and cooked partially, about 2 to 3 minutes per side.
Remove pork to a plate, return skillet to stove, and add garlic. When garlic is fragrant, add green beans and stir occasionally until browned, about 3 minutes. Add sherry and hoisin, stir to combine, and cook until beans are tender and sauce is reduced slightly and sherry smell is cooked off, about 4 minutes. Return pork to pot, turn to coat in sauce, season with salt and freshly ground black pepper. Sprinkle with sesame seeds and green onions, if using, and serve over rice.