Place cashews in a bowl with hot water to cover; soak for 30 minutes. Drain.
Combine butternut squash and onion in a pot with water to cover by an inch. Bring to a boil. Reduce heat to medium; simmer until tender, about 15 minutes.
Bring a large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water, stirring occasionally, until tender yet firm to the bite, about 8 minutes. Reserve 1/4 cup of the cooking liquid; drain the rest. Place macaroni back in the pot.
Combine the cashews, squash, onions, and reserved cooking liquid in a blender. Add lemon juice, salt, mustard, garlic powder, black pepper, paprika, nutmeg, and turmeric. Blend until completely smooth. Pour sauce into the pot of macaroni and stir to coat. Warm through if needed.