Preheat oven to 400 degrees Fahrenheit, and cut the acorn squash in half lengthwise. Brush some olive oil on the flesh, and sprinkle with salt and pepper. Please each half, cut side up, on a baking sheet lined with parchment paper, and bake for about 40 minutes, or until squash is tender. Remove squash from oven, and set aside to cool. 

While the squash is roasting, sautée the shiitakes in a frying pan on medium heat, with a drizzle of olive oil. Add salt and pepper, and remove mushrooms from the heat after about 5 minutes, and they’re nice and brown. Spoon mushrooms into a bowl and set aside. 

In a large cast-iron skillet, heat some olive oil over medium heat. Bring the vegetable broth to a boil in a medium saucepan, and then reduce to a low simmer. 

Add the chopped onion and celery to the skillet, and cook until fragrant, about five minutes. Add the garlic and thyme, and cook for another 2-3 minutes. Add rice to the skillet, and toast for about 1-2 minutes. 

Now, stir the broth into the rice, 1-2 ladlefuls at a time, although this will depend on the size of your ladle. The rice should be just covered with liquid, and the broth should be simmering lightly, but not boiling. Adjust your heat accordingly. Once the broth has been completely absorbed, replenish the liquid. Repeat this process about 4 times or until all of the broth has been used up. The whole process should take about 30 minutes, and when finished, the rice should be tender and all the liquid should be gone.

While the risotto is cooking, use a food processor to puree the squash. Add up to a quarter cup of milk or water if you’re having trouble getting it to the right consistency. Once the risotto is finished cooking, stir in the squash, parmesan cheese, and half of the mushrooms. Add salt and pepper as needed. 

Transfer the risotto to serving dishes and top with the remaining shiitakes and additional parmesan cheese if you’d like.