Shave the stem and the tip off each zucchini (or other summer squash), scrub the vegetable thoroughly but not harshly with a brush under cold running water to remove any clinging sand or dirt. If vegetables are large, halve or quarter them. If seeds are large and at all tough, and surrounding flesh is coarse rather than moist and crisp, which is more often the case with yellow squashes and striped green cocozelles than with zucchini, cut out and discard the cores.
Rub the squash against the coarse side of a grater, and place grated flesh in a colander set over a bowl.For each 1 pound (2 cups) of grated squash, toss with 1 teaspoon of salt, mixing thoroughly. Let the squash drain 3 or 4 minutes, or until you are ready to proceed.
Just before cooking, squeeze a handful dry and taste. If by any chance the squash is too salty, rinse in a large bowl of cold water, taste again; rinse and drain again if necessary. Then squeeze gently by handfuls, letting juices run back into bowl. Dry on paper towels. Zucchini will not be fluffy; it is still dampish, but the excess liquid is out. The pale-green, slightly saline juice drained and squeezed out of the zucchini has a certain faint flavor that can find its uses in vegetable soups, canned soups, or vegetable sauces.
While the shredded zucchini is draining (reserve the juices,) drop the rice into boiling salted water, bring rapidly back to the boil, and boil exactly 5 minutes; drain and set aside.
In a large (11-inch) frying pan, cook the onions slowly in the oil for 8 to 10 minutes until tender and translucent. Raise heat slightly and stir several minutes until very lightly browned.
Stir in the grated and dried zucchini and garlic. Toss and turn for 5 to 6 minutes until the zucchini is almost tender.
Sprinkle in the flour, stir over moderate heat for 2 minutes, and remove from heat.
Gradually stir in the 2 1/2 cups warm liquid (zucchini juices plus milk, heated gently in a pan -- don't let it get so hot that the milk curdles!). Make sure the flour is well blended and smooth.
Return over moderately high heat and bring to the simmer, stirring. Remove from the heat again, stir in the blanched rice and all but 2 tablespoons of the cheese. Taste very carefully for seasoning.
Turn into a heavily buttered 6- to 8-cup, flameproof baking and serving dish about 1 1/2 inches deep, strew remaining cheese on top, and dribble the olive oil over the cheese.
About half an hour before serving, bring to simmer on top of stove (you can skip this step if your baking dish isn’t flameproof), then set in upper third of a preheated 435-degree F oven until tian is bubbling and top has browned nicely. The rice should absorb all the liquid