Trim most of the stem off the shiitake (they tend to be very woody).
Heat a sauté pan over medium high heat and add avocado oil. When hot, add the mushrooms and cook until browned, about 10 minutes.
Add all remaining ingredients except sesame oil, green onion, and sesame seeds. Boil over high heat to reduce sauce to form a thick glaze. Add a bit of water if the sauce seems too thick. Adjust seasoning to taste with salt, sweetener, and sriracha and stir in the toasted sesame oil. Garnish with green onions and sesame seeds.