Prepare butternut squash: Preheat oven to 350 F. Cube butternut squash (removing seeds and skin), toss with oil, salt and pepper and roast for 45-60 minutes, flipping halfway through. Squash is done when the edges start to brown slightly. The squash will be soft like a sweet potato. 

Prepare pasta and sauce components: Start boiling salted water in a pasta pot and prepare pasta according to package instructions. When pasta is finished, reserve 2-3 cups of pasta water. On another burner, boil cashews for 15 minutes. If finished before pasta and butternut squash, leave in warm water until ready to blend. Meanwhile, sauté thinly sliced shallot in a frying pan with a tablespoon of oil for 2-3 mins. Add minced garlic and cook for another minute, then set aside off the heat.

Blend sauce: combine warm butternut squash, warm pasta water, cashews, shallot/garlic mixture, and remaining sauce ingredients until smooth (2-3 minutes in a high speed blender, longer in other blenders). 

Optional - sauté mushrooms: in the same frying pan, add a tablespoon of oil on medium heat and sautee sliced scallions or extra shallots for 2-3 minutes. Then add thinly sliced mushrooms and cook until they release juices. Add a tablespoon of soy sauce and cook for another minute or two until mushrooms reabsorb juices. 

Mix it all together: in the empty pasta pot, combine pasta, half the sauce, and mushroom mixture. Stir until sauce is well distributed, adding more sauce if needed. Serve immediately.