2lbszucchini or summer squash, ~4 medium or 6 small squash
1/2cupwhite or jasmine rice (not quick-cook or instant)
3TBolive oil (divided)
2tspgarlic powder (or 2 garlic cloves, minced)
2TBflourUse AP gluten-free flour if needed
2cupsliquid (zucchini juices plus rice water)See Recipe Notes
2/3cupParmesan, freshly grated (divided)Or substitute Swiss, Jarlsberg, or Smoked Gouda
1lemon, cut into wedges
Preheat oven to 425°F.Grate all of the zucchini. Use the large side of a box grater or finely chop in a food processor.Place the grated zucchini in a mesh sieve, and then set the sieve on top of a large bowl. Stir the salt into the zucchini, and let it rest about 4-5 minutes. As it rests, the zucchini will start to sweat, releasing liquid into the bowl below.
Squeeze the zucchini to release the rest of the liquid. The zucchini will still feel damp to the touch, but most of the liquid has been released. Reserve the liquid.
Lay the grated zucchini out on a towel to dry.
Bring a medium pot of water to a boil. Add the rice, and cook for exactly 5 minutes.After 4 minutes, begin ladling water out of the pot and into the bowl with the zucchini juice until you have a total of 2 cups of liquid. It's okay if you get a little bit of rice mixed in with the liquid.After 5 minutes, strain the rice and discard the remaining liquid.
Heat 2 TB oil over medium heat in a large, oven-safe pan (like cast iron or carbon steel).Add the diced onion to the pan, and cook about 3-4 minutes, until the onions begin to brown.
Add the zucchini and garlic to the pan with the onions. Cook 3-4 minutes, until the zucchini is tender.
Add the flour to the zucchini mixture, and stir. Cook about 2 minutes until the flour is incorporated. Stir well and make sure there are no lumps of uncooked flour before proceeding.
Slowly pour the warm zucchini + rice water mixture into the pot, stirring as you pour. Continue cooking until the liquid begins to simmer.
Stir in the rice and 1/2 cup of the cheese. Sprinkle the remaining cheese across the top of the zucchini mixture, and drizzle with the remaining 1 TB oil.Make Ahead Tip: If desired, you can stop at this point and finish cooking later. Make up to one day ahead, and then reheat to a simmer before proceeding to the next step.
Carefully move the pan into the oven. (It will be very full at this point!)Bake about 30 minutes, until the cheesy top has browned and the rice has absorbed the liquid.Squeeze a little fresh lemon juice over the vegetables just before serving. Serve immediately.
Gratin liquid options: As written, this recipe uses the zucchini juices plus starchy rice water. If desired, you can swap the liquid for warmed milk or broth.