In a large pot of boiling water, cook ramen noodles (2 3.5-ounce packages) until tender, about 3 to 4 minutes; rinse with cold water and drain well.
In a medium bowl, whisk together beef stock (1/3 cup), oyster sauce (1/4 cup), rice wine vinegar (1 tbsp) and Sriracha (1 tsp).
Heat sesame oil (1 tbsp) in a large skillet over medium heat. Add ground beef (1 lb) and sweet onion (1 cup), and cook until beef has browned, about 3 to 5 minutes, making sure to crumble the beef as it cooks; drain excess fat.
Stir in garlic (3 cloves) and ginger (1 tbsp) until fragrant, about 1 minute.
Stir in beef stock mixture, scraping any browned bits from the bottom of the skillet.
Stir in ramen noodles until heated through and evenly coated in sauce, about 1 to 2 minutes.
Serve immediately, garnished with green onions (2) (optional) and sesame seeds (1/2 tsp) (optional).