Spread sliced onion in a microwave-safe dish. Cover loosely with plastic wrap and microwave for 1 minute 30 seconds at 600W. Stir and flatten again, then microwave for another 2 minutes. Let cool and drain any moisture.
Cut cucumber into thin diagonal slices, then julienne. Slice chikuwa diagonally into 7mm thick pieces.
Rehydrate hijiki in water, boil briefly, and drain.
Combine all ingredients in a bowl and toss with sesame garlic dressing. Chill before serving if making ahead.