In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté for about 3-4 minutes until softened.
Add the minced garlic and diced bell pepper to the skillet, cooking for an additional 2-3 minutes until the bell pepper is tender.
Increase the heat to medium-high and add the ground beef to the skillet. Cook, breaking it apart with a spatula, until browned and cooked through, about 5-7 minutes. Drain any excess fat.
Stir in the soy sauce, hoisin sauce, sesame oil, ground ginger, and red pepper flakes (if using). Mix well and let it simmer for 2-3 minutes to combine the flavors. Season with salt and pepper to taste.
To serve, spoon the beef mixture into the lettuce leaves and garnish with sliced green onions and sesame seeds.