Bring a large pot of salted water to a boil. Add the cauliflower and cashews and boil until the cauliflower is fork-tender, about 8 minutes. Drain and transfer to a blender. Add the olive oil, water, garlic, lemon juice, nutritional yeast, salt, mustard, and several grinds of pepper. Blend until creamy.
Cook the pasta in a large pot of salted boiling water according to the package directions, or until al dente. Drain the pasta, reserving 1 cup of the starchy pasta water.
Transfer the pasta back to the pot, pour the sauce over the pasta and toss to coat. If the sauce is too thick, add some of the reserved pasta water until it’s creamy.