In a small bowl, combine all the stir-fry sauce ingredients and set aside. Slice the mushrooms into half-inch thick pieces to ensure even cooking. Keep them aside until ready to stir-fry.

Heat a large wok or pan over medium-high heat and drizzle in the oil. Add the sliced mushrooms and sauté for 2-3 minutes, stirring occasionally to ensure even browning. Sprinkle a pinch of salt to enhance their natural flavor. Once the mushrooms start to turn golden and their internal moisture evaporates, add the finely chopped garlic and sliced onions. Stir well and let them cook until the onions soften and become fragrant.

Increase the heat to high and add the noodles to the wok. Pour the prepared sauce over the noodles, ensuring they are evenly coated. Stir-fry everything for 1-2 minutes, tossing continuously until the noodles absorb the sauce and develop a deep, glossy color. The sauce should be nearly dry by the time they are ready.

Add the chopped spring onions and give everything a final toss to distribute the flavors evenly. Remove the wok from the heat and serve the Mushroom Chow Mein immediately while hot.