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1
Heat the oven to 375F.
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2
Cut the spaghetti squash in half length-wise and scoop out the seeds.
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3
Brush the flesh with 2 tablespoons of olive oil and sprinkle with a little salt and pepper.
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4
Place the squash, cut side down, on a rimmed baking sheet and bake until tender, about 45 minutes.
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5
Remove the squash from the oven and let it stand until it is cool enough to handle.
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6
Scrape the flesh into a bowl and set aside.
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7
Leave the oven on.
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8
While the squash is baking, bring the vegetable broth to a boil in a saucepan placed over medium-high heat.
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9
Carefully pour in the rice.
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10
When the liquid returns to a boil, reduce the heat to medium-low and cover.
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11
Cook the rice for about 15 minutes, until the grains are slightly underdone and the liquid has been absorbed.
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12
Transfer the rice to a large bowl and toss with 1 tablespoon of olive oil.
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13
Put the remaining 3 tablespoons of oil and the garlic in a large skillet or saute pan and set the pan over medium-low heat.
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14
Saute the garlic until it is softened but not at all browned, about 5 minutes.
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15
Add the mushrooms, onion, and zucchini and sprinkle with a little salt and pepper.
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16
Saute the vegetables for about 10 minutes, until they are tender and the liquid they release has been reabsorbed.
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17
Stir in the parsley.
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18
Remove from the heat and transfer to a bowl.
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19
Let cool for a few minutes.
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20
Add the reserved spaghetti squash and the mushroom-zucchini mixture to the bowl with the rice.
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21
Add the dry Jack and mozzarella cheeses and gently but thoroughly fold everything together.
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22
Grease an 8-by-12-inch or similar-sized gratin dish with a little olive oil.
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23
Pour the rice-vegetable mixture into the dish and sprinkle the Parmigiano on top.
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24
Bake the gratin for 20 to 25 minutes, or until the top is golden brown and the edges are crusty.
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25
Remove the gratin from the oven and let it rest for 5 to 10 minutes before serving.
