Fill a steamer pot or saucepan with water to just below the base of a steamer basket. Bring water to the boil.
Cut broccoli into small florets, trimming stalks to about 2 cm. Peel remaining stalk and cut diagonally into thin slices. Trim buk choy and slice stems. Trim spring onions and slice thinly on the diagonal.
Combine broccoli, buk choy, spring onion and sugar snap peas in a large bowl. Add garlic and ginger; toss well. Transfer to steamer basket. Steam, covered, until vegetables are crisp-tender, 3 to 4 minutes.
Place vegetables in a serving dish. Combine fish sauce and sugar in a small cup, stirring until sugar has dissolved. Drizzle over the vegetables. Serve hot or chill briefly in the refrigerator before serving.