Heat the vegetable oil in a large skillet over medium heat.
Add the chopped onion and garlic to the skillet; sauté for 3-4 minutes until translucent.
Stir in the ground chicken, breaking it apart with a spatula; cook until browned.
Add the chopped bell pepper and mushrooms; cook for an additional 5-7 minutes, stirring occasionally.
Pour in the soy sauce and hoisin sauce; mix well and simmer for 2-3 minutes.
Add the sesame oil and chopped green onions; stir to combine and remove from heat.
Separate lettuce leaves and wash gently; pat dry with a towel.
Spoon the chicken filling into each lettuce leaf; serve immediately.