In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes.

Add the minced garlic and diced bell pepper to the skillet, cooking for an additional 2-3 minutes until softened.

Increase the heat to medium-high and add the ground beef. Cook until browned, breaking it apart with a spatula, about 5-7 minutes. Drain excess fat if necessary.

Stir in the chopped mushrooms, soy sauce, hoisin sauce, ground ginger, and sesame oil. Cook for another 3-4 minutes until the mushrooms are tender and the mixture is heated through. Season with salt and pepper to taste.

To serve, spoon the beef mixture into the lettuce leaves and garnish with chopped green onions and sesame seeds if desired.