To prepare the mushrooms, clean them with a brush or paper towel to remove debris.
Prepare a bowl with ice water and set it aside. Then, fill a wok or large skillet with water with ¼ - ½ teaspoon of salt, and bring it to a boil. Cook the mushrooms for about 10 minutes. The mushrooms should have a glossy outer layer once cooked. Remove and soak it in the ice water bath to stop the cooking.
Before cutting the mushrooms, pat dry with a cloth or paper towel to remove moisture. Give them a gentle squeeze if needed.
Next, prepare the marinade by combining soy sauce, vegan oyster sauce, and pepper in a large shallow plate or baking pan.
To cut the mushroom into a cutlet shape, place it on a sturdy work surface. Using a sharp knife, make a slit from the cap to the stem. Slice it in a spiral motion like how you peel an apple to get a 1cm thick piece. Then, gently score both sides so they better absorb the sauce.
Transfer mushrooms cutlet to the marinade, then rub both sides generously with the liquid. Marinate for 15-20 minutes.
Meanwhile, whisk together tempura batter mix with water until no large lumps appear.
Now, fill a large wok with 5-6 cups of oil and turn on the heat. Once the oil is heated to about 350F, dredge a mushroom cutlet in the batter and slowly slide it into the oil. Fry both sides until golden brown, about 8-10 minutes, with a flip if necessary. Remove and transfer to a drain rack to cool down a little before cutting. The drain rack will keep it crispy while you make the sauce.
For other ways to cook the mushroom cutlet, please refer to the '3 ways to cook the mushroom' section.
To make the lemon sauce, whisk together the sauce ingredients in a bowl (except lemon slices) until the sugar has dissolved. Pour the sauce in a heated small skillet and bring it to a boil. Once the sauce starts to simmer, add in the lemon slices and continue to cook the sauce until thickened. The sauce will start to thicken as soon as you see large bubbles. Turn off the heat and spoon this sauce over the cut mushrooms cutlet. Serve warm with rice or as-is.