Heat the oil in a large skillet. Add the onions, garlic, ginger, and salt. Saute for 5 to 6 minutes. Add the water and cauliflower, cover the pan, and steam the vegetables for 3 minutes. Then add the broccoli, cover again, and steam for 4 minutes longer. The broccoli should be bright green and tender. Add the lemon juice to the vegetables just before serving. If you wish, sprinkle lemon zest over the vegetables as a garnish.