Start heating the oil in a medium size pan or skillet over medium heat (to ~350 - 360º F).
King Oyster Mushrooms - remove the caps and shred the stems by holding one end and pulling a fork through the length of the stem. Turn the stem and continue to shred until you've shredded the whole stem. Pull any bigger pieces apart.
OR for larger chunks, break the stems into long thin pieces instead of shredding.
Save the caps for another recipe or break into chunks and fry along with the rest.
Tempura Batter: sift all dry ingredients into a medium bowl. *Add 1 cup of cold water, stir, and then add more as needed to resemble thin pancake batter.
*Don't add the cold water to the dry ingredients until the oil is hot and the mushrooms are ready to go.
Check to be sure the oil is hot. If no thermometer, place drop of batter into the pan - it should sink down, bounce up, and then sizzle and bubble right away.
Once the oil is ready, add the first batch of mushrooms to the pan. Pro Tip: At least half the surface should be oil (don't overcrowd the pan).
After 2 or 3 minutes, gently flip the mushrooms (away from you) and cook the other side until golden and crispy.
Remove to cooling rack with paper towel below to catch the drippings. Immediately sprinkle with salt so it sticks to the hot tempura. Continue until all the mushrooms are cooked.
Remove all the stray bits of tempura from the oil before each new batch so you don't get burnt bits sticking to next batch.