Preheat the oven to 400° F
Shred the zucchini and toss them, along with a heaping tsp of salt and allow to drain in a colander (put a bowl under the colander as you'll want to reserve the juices).
After 10-15 minutes, squeeze as much as you're able, of the excess water out of the zucchini. (I've found if I put the zucchini in a nut bag
, it works really well)
Set the zucchini aside on paper towels to dry
Meanwhile, bring a small pot of water to a boil.
Drop the rice into the boiling (and salted) water. Bring it back to a rapid boil for 8 minutes.
Drain, run cold water over the rice, and set aside on paper towels to dry
Return the pan to the stovetop over low (or simmering) heat. Keep an eye on it so it doesn't burn, we're simply warming it a bit while we work on browning the onions and zucchini
Add 1 part zucchini juices to 1 part milk. (The recipe calls for 2 ½ cups of liquid in total. If you'd like your gratin to be on the wetter side, I'd lean toward 2 ½ cups. For a drier gratin, closer to 2 cups)
In a large frying pan over medium heat, cook the onions slowly in 3 to 4 Tbsp. olive oil for 8 to 10 minutes, or until they're tender and translucent.
Raise the heat to medium-high and continue to cook, stirring slightly for several minutes, or until the onions have just browned
Add the zucchini and garlic to the pan, toss, and cook until the zucchini is almost tender.
Sprinkle in the flour, stir and remove from the heat after a minute or so
Gradually stir in the 2 ½ cups warm milk/zucchini juice; make sure the flour is well blended and smooth.
Put the pan over moderately high heat and bring to a simmer, stirring.
Remove from the heat again, stir in the blanched rice and ½ cup of the grated Parmesan.
Taste carefully for seasoning and add additional salt and freshly ground black pepper if necessary
Butter a 6-8 cup baking dish that is about 1 ½ to 2 inches deep.
Turn the zucchini mixture into the prepared pan and sprinkle the remaining Parmesan cheese on top.
Dribble the remaining olive oil over the cheese.
Bake in the upper third of the oven until the gratin is bubbling, the top has browned, and it appears that most of the liquid has absorbed. (Keep an eye on it, and you'll begin to see it set up, and the surface become slightly browned) Mine took about 30-35 minutes
Let it sit for about 8 to 10 minutes before serving. It will still be hot, and the dish will hold together a bit better for serving.