Preheat oven to 425°.
Wash the mushrooms, pat dry, and remove the stem and the gills (I used a small spoon to remove the mushroom gills).
Coat mushrooms in avocado oil, place on a baking sheet and cook in oven for 15 minutes.
While mushrooms are roasting, cook the ground chicken with salt and pepper on medium-high heat for 4-5 minutes.
Add 1 Tbsp sesame oil and chopped bell peppers to skillet with ground chicken and cook for additional 5-6 minutes until chicken is cooked through and peppers softened.
Add water chestnuts to the skillet and cook for 1-2 minutes, then add chives.
While the chicken and vegetable mixture is finishing up cooking, make the sauce. Combine sesame oil, coconut aminos, rice wine vinegar, maple syrup, Chinese five spice, chopped ginger, and minced garlic in a bowl.
Pour sauce into the skillet, stir to combine and remove from heat.
Remove mushrooms from the oven and set aside to cool.
Spoon chicken mixture into portabella mushroom caps, top with additional chives if desired and enjoy!