Wash the zucchini, pat them dry, cut off the ends, then cut them lengthwise into slices approximately ½ cm thick and set aside. Delicately clean the mushrooms by rubbing them with a cotton cloth, then slice them. Clean the onions, slice them and put them in cold water to soak. Wash the parsley leaves, pat them dry and then chop them very coarsely. Grease a griddle (or a heavy-bottomed grill pan) with half a spoonful of oil, put it on the stove and, when its nice and hot, grill the slices of zucchini and mushroom on both sides. Arrange half of the ingredients on a serving dish in the following order: grilled zucchini, grilled mushroom, sliced spring onion (well drained and patted dry), parsley and grated lemon zest. Dress with a spoonful of olive oil and season with salt and pepper. Add another layer using the remaining ingredients in the same order; finish off with a good drizzle of oil, a pinch of salt and some freshly ground pepper. Before serving, leave to rest for about a quarter of an hour.