In a small bowl or measuring cup, whisk together the maple syrup, soy sauce, rice vinegar, sesame oil, red pepper flakes (if using), and cornstarch until smooth. Set aside.
Heat olive oil in a large skillet over medium-high heat. Add the ground beef and cook, breaking it up with a spoon, until fully browned, about 5 minutes.
Stir in the garlic and ginger and cook for 1–2 minutes, until fragrant.
Add the diced bell pepper and shredded carrots. Cook for 3–4 minutes, until vegetables are tender but still slightly crisp.
Pour the sauce into the skillet and stir well. Simmer for 2–3 minutes, until the sauce thickens and coats the beef evenly. Season with salt and black pepper as needed.
Stir in the sliced green onions and remove from heat.
Spoon the beef mixture into lettuce leaves. Garnish with sesame seeds and serve with rice or cauliflower rice if desired.