Prepare the mushrooms. Clean the stems with a brush to remove debris. Then, cut the cap off and reserve for another dish, then halve the stems, crosswise to get two logs.
Bring a pot of water to boil and add with a pinch of salt. Boil the mushroom stems until they turn slightly softer, from white to grayish, about 3-5 minutes. Remove and let it cool, or soak in an ice water bath to speed up the process.
While waiting, prepare the marinade in a bowl. Once the mushrooms are cool to handle, pat them dry with a towel. Cut each stem in half, lengthwise to get 2 cutlets.
Lay the flat side on a work surface. Use a meat tenderizer and gently pound it a few times. Lay the cutlets on a baking pan or shallow plate and pour the marinade over, then rub them generously until well coated. Marinate for 10 minutes.
Next, combine crispy tempura mix and enough ice-cold water to create a thick batter (I use a little more than ½ cup of water) in a shallow plate.
Heat a 10" skillet with enough oil to cover the entire bottom of the pan (about 0.5 cm rise to the side) over medium heat.
Dip mushroom cutlets in the batter then sprinkle generously with sesame seeds. Pat to adhere. Slowly lower the cutlets into the hot oil. Add as many cutlets as the pan comfortably fits in one single layer.
Shallow fry until golden brown before flipping. Continue to shallow fry this side for another 1-2 minutes. Then, transfer the cutlets to a paper-lined plate to absorb excess oil.
Slice the cutlets into two and serve warm with a side of your favorite condiment.