Heat olive oil in a large skillet over medium-high heat.
Add ground beef and cook until browned, breaking it apart as it cooks. Drain excess fat if needed.
Add the onion and garlic, and sauté for 1-2 minutes until fragrant.
Toss in the bell peppers and stir-fry for about 4-5 minutes until slightly tender but still crisp.
Stir in soy sauce and hoisin sauce, then pour in the cornstarch-water mixture.
Cook for another 2-3 minutes until the sauce thickens and coats the beef and veggies evenly.
Season with salt and pepper to taste.
Garnish with chopped green onions and sesame seeds if desired. Serve hot over rice or noodles.