Trim any tough stems or growing medium off the base of the mushrooms. If the mushrooms form clusters like enoki, maitake, or shimeji, be sure to leave enough stem intact, so they don't fall apart.
Break any clusters of mushrooms up into smaller clusters, as shown.
Trim the stem end off the green beans. Pluck some basil leaves from the stems, and make sure they are dry.
Dust any smooth surfaced vegetables such as green beans in flour, or starch.
Heat 2-inches of oil in a heavy-bottomed pot or pan until it reaches 340 degrees F (170 C). Prepare a paper towel-lined rack to drain the tempura mushrooms.
Add the cold water into a bowl and then add the flour. Give it a few stirs to form the batter, but do not overmix it. There should still be a lot of pea-sized lumps of flour in the batter.
When the oil is heated, fry the basil first in small batches. It will bubble up, so don't add too many at a time, or your pot will overflow. The basil is crisp when there are almost no bubbles coming out of it. Transfer the basil to the rack.
For the green beans, dip them in the batter to coat evenly and lower them carefully into the oil. Fry them until they are vibrant green and tender, but not so long that the batter starts to brown. Transfer the beans to the rack to drain.
Be sure to skim off any crumbs of batter each time before you add the next ingredients to the oil.
For clusters of mushrooms (like enoki or maitake), grab a cluster by the stem end and dip the whole thing in the batter. Let any excess batter run off before you add it to the oil. As you lower the cluster into the oil, start shaking it from side to side. Be careful not to splash hot oil on yourself.
Fry the clusters of mushrooms, flipping them over at least once until they hold their shape, and they're crisp. Transfer them to the rack.
For large mushrooms such as shiitake, dip the underside of the cap in the batter, but leave the top of the mushroom unbattered. Lower the half-battered mushrooms into the oil.
Fry the shiitakes until they're crisp on the bottom and then transfer them to the rack.
Serve the Tempura on a sheet of paper, or you can use paper towels or napkins along with good finishing salt.