In a wok or large skillet, heat 1 tbsp of the vegetable oil over high heat; stir-fry beef in 2-3 batches, until browned but still pink inside, about 2 minutes. Transfer to a plate. Drain fat from pan.
Add remaining vegetable oil to pan; stir-fry sliced onion and minced garlic over medium heat until onion is softened.
Stir in sweet potato, red or green pepper and 1/2 cup stock; cover and steam until potatoes are tender.
Meanwhile, in a small bowl, whisk together the remaining 1/2 cup stock, oyster sauce, corn starch, vinegar and sesame oil. Return beef and any juices to pan. Add oyster sauce mixture and bring to a boil; boil, stirring until thickened and glossy. Sprinkle with green onions.