If you are using dried shiitake mushrooms, soak them in warm water for 10-15 minutes, or until they are soft. Then cut them into small pieces.
Drain water from canned water chestnuts and dice water chestnuts into small pieces. Mince onions and garlic. Slice green onions.
Combine all the sauce ingredients in a small bowl and mix well.
Add vegetable oil to a large skillet and heat skillet. Add garlic and onions. Sauté briefly until fragrant - do not brown or burn garlic and onions. Quickly add ground pork and break pork into small pieces. Season with a few pinched of salt and pepper.
Once pork is cooked and no longer pink-ish, add shiitake mushrooms, water chestnuts, and sauce. Stir to combine and bring to a boil.
Cook for another 1-2 minutes, or until liquid thickens. Remove from heat.
Fill lettuce with pork filling. Garnish with green onions and red pepper flakes (optional). Serve and enjoy!