1. Warm 1 Tbsp. oil in a large nonstick skillet over medium-high heat. Add mushrooms and cook, stirring occasionally, until golden brown, 8 to 10 minutes.
2. Stir in remaining oil, garlic, ginger and white parts of scallions. Cook, stirring constantly, until fragrant, about 30 seconds. Add pork and sauté, breaking up meat, until just cooked through, about 3 minutes longer.
3. Add carrot and bell pepper to skillet and sauté until vegetables begin to soften, about 3 minutes. Stir in rice, water chestnuts, soy sauce and sesame oil. Cook, stirring constantly, until rice is coated and heated through, about 3 minutes. Turn off heat and stir half of reserved scallion greens into pork mixture.
4. Divide pork mixture among lettuce leaves; sprinkle with additional scallion greens.