Fit a steamer basket into a medium-sized saucepan with a tight-fitting lid. Add 2 to 3 inches of cold water to the pot and then add the butternut squash and garlic. Cover and bring to a boil. Steam the butternut squash and garlic until very tender, about 20 minutes. Let the butternut squash and garlic cool for about 20 minutes (see note).
Meanwhile, put the cashews and ½ cup water in a small bowl. Let soak for 30 to 40 minutes. Drain the soaking water from the cashews and thoroughly rinse the cashews under clean, cold running water.
Once the butternut squash and garlic have cooled, bring a large pot of water to a boil over medium-high heat. Add ¼ teaspoon salt (optional). Stir in the pasta. Decrease the heat to medium-low and cook, stirring occasionally, until the pasta is almost tender. Add the peas and cook, stirring occasionally, until the pasta is tender but firm and the peas are heated through, about 3 minutes. Drain the pasta and peas.
While the pasta and peas cook, put the cooled butternut squash and garlic, cashews, 1½ tablespoons of cold water, mustard powder, smoked paprika, ½ teaspoon salt, cayenne pepper and turmeric into a blender and process until the consistency of a thick, smooth sauce. Add more water 1 tablespoon at a time, as needed, to achieve the desired consistency.
Transfer the sauce to a medium-sized pan and cook over medium-low heat, just until heated through, stirring often and adding more water as needed if the sauce is too thick.
Put the pasta and peas in a large bowl. Immediately pour the warm sauce over the pasta and gently stir to combine. Season with salt and pepper, to taste. Serve hot.
Chef’s note: The butternut squash and garlic may be steamed in advance of preparing this recipe. After steaming, cool and store tightly covered in the refrigerator for up to 24 hours.