Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Cover and bring the water to a boil over high heat. Arrange the broccoli and silverbeet on the steamer, recover and steam for 5 to 6 minutes.
Add the peas, spinach and garlic; continue to steam until the vegetables are tender, about 5 minutes more. Remove the vegetables from the steamer and reserve 1/2 cup of the hot water.
Place the vegetables, hot water and lemon juice in a blender. Hold down the lid of the blender with a folded kitchen towel and carefully start the blender, using a few quick pulses to get the vegetables moving before leaving it on to puree.
Cool before serving or divide the puree into four equal portions and freeze in resealable plastic bags for future use.