Slice king oyster mushrooms lengthwise and diagonally. Remove stems from shiitake mushrooms and slice thinly.

Julienne the sweet potato and soak in water to remove excess starch. Drain well.

Combine sliced ingredients, soy sauce, salt, and pepper in a microwave-safe container. Add butter, cover loosely with plastic wrap, and microwave at 600W for 6–7 minutes.