Slice the beef or venison against the grain into ¼ inch strips. Then cut the strips into 2 inch pieces.
In a Ziploc bag, add the cornstarch and the sliced meat. Shake the bag and massage the cornstarch into the meat, making sure to cover the meat completely.
In a wok or large skillet, heat vegetable oil at medium-high heat until it is shimmering. Add meat in batches, being careful not to crowd the pan. Cook until meat is nicely browned. Remove cooked meat to a bowl lined with paper towels.
In a medium saucepan, add soy sauce, water, oil, vinegar, brown sugar, garlic, ginger, and red pepper flakes. Bring the sauce to a boil. Then lower to medium heat and simmer, allowing the sauce to thicken. This should take 20 minutes.
Add all the meat back to the wok or skillet and pour the sauce over the meat, stirring to coat all the meat. Cook on medium for about 5 minutes.
Serve Mongolian beef over rice and garnish with thinly sliced onions.