In a 5- to 6-quart pan over high heat, bring 2 1/2 to 3 quarts water to a boil. Add soba and stir occasionally until barely tender to bite, 5 to 7 minutes. Drain, rinse with cold water until cool, and drain again thoroughly.
Meanwhile, in a small bowl, mix wasabi powder with 1 tablespoon cold water; let stand about 5 minutes. Add soy sauce and lemon juice.
In a large bowl, mix spinach with 2 tablespoons of the wasabi dressing. Divide spinach among dinner plates.
Add noodles, green onions, and remaining dressing to bowl; mix gently. Mound equally on spinach and top with equal portions of shrimp and ginger.
Four types of noodles: Look for dried Asian noodles in the international section of the supermarket or in an Asian market.
Bean threads (saifun or cellophane noodles). Thin, wiry dried noodles, made from the starch of mung beans, turn clear and slippery when cooked in water or puffy and crisp when deep-fried. Neutral flavor.
Rice noodles (rice sticks, mai fun, mi fun). Dried white noodles, made from rice flour, vary from whisker-thin to about 1/4 inch wide. When cooked in water, they turn opaque and tender; when fried, they puff and crisp. Mild rice flavor.
Soba. Buckwheat and wheat flour infuse thin, tan Japanese noodles with robust, earthy flavor.
Wheat noodles (Chinese noodles or Oriental noodles, mein). Available in many forms, these all-purpose noodles taste similar to spaghetti and go by many names.