Prepare the ingredients. Quarter the shiitake mushrooms and chop the bell peppers into similar sizes. Finely chop the onion and prepare the sauce.

Beat the egg well. Place the shiitake mushrooms in a plastic bag, add the egg mixture, and shake to coat. Then add potato starch and shake again to coat evenly.

Heat grapeseed oil in a frying pan. Add the coated mushrooms, cover with a lid, and fry until golden brown, flipping once. Remove and drain on a rack.

Combine the sauce ingredients and chopped onion in a pan. Simmer over medium heat until it thickens into a syrup-like consistency.

Add the fried mushrooms and bell peppers to the sauce and mix well.

Heat for 1–2 minutes to meld flavors, then remove from heat.