Separate the onion into rings. Steam the broccoli 5 minutes (or microwave a few minutes).

In an electric blender combine the flour, salt, baking soda, egg yolk, and water. Blend to mix. Let stand 15 minutes.

Heat 3 - 4 inches of oil in a deep heavy kettle, deep-fat fryer, or electric wok until it registers 375F / 190C on a deep-fat thermometer. Test batter consistency by dipping one piece of vegetable and letting excess drip off. There should be a light coating left on.

Dip and fry, a few at a time, in the hot oil until golden. Drain on paper towels and keep warm in the oven heated to 250F / 130C / Gas Mark until all are cooked.