Bring a large pot of salted water to a boil (about 3-4 quarts water, 1-2 tbsp salt). Add the spaghetti and cook to al dente or your desired doneness. Reserve about 1 ½ cups of pasta water, then drain and transfer back to the pot.
In a large skillet, warm the oil over medium low heat. Add the shallot, red pepper flakes or black pepper (if using), and ¼ teaspoon salt and cook for 3-5 minutes, or until softened, stirring occasionally.
Add the cauliflower and remaining ½ teaspoon salt, increase the heat to medium-high, and cook for 5 more minutes, stirring occasionally.
Transfer the cauliflower and shallots to a high-speed blender along with the hemp seeds, nutritional yeast, lemon, miso, and 1 cup of the reserved pasta water. Blend until totally smooth. If the sauce is too thick to blend, add a splash of pasta water or warm tap water to thin it out.
Pour the sauce into the pot (scrape every last bit) and toss with the pasta. As the pasta sits, it will soak up that sauce and get a little dry. It’s still delicious but you can pour in more pasta water or warm tap water and toss again to thin it out.
Serve right away topped with some black pepper and fresh parsley (if using). If you have extra bread crumbs from this week’s kale salad, those are delicious on top and add a nice crunch!