Soak the Cashews: Place the raw cashews in a bowl and cover them with boiling water. Let them soak for at least 15 minutes, or up to 2 hours. This step is crucial for making them soft enough to blend into a smooth, creamy sauce.

Cook the Pasta: While the cashews are soaking, bring a large pot of salted water to a boil. Cook the pasta according to package directions until al dente. Before draining, reserve about 1 cup of the hot pasta water.

Sauté Aromatics: In a large skillet or saucepan, heat the olive oil over medium heat. Add the minced garlic and chopped shallot and cook for 2-3 minutes, until softened and fragrant. Add the red pepper flakes (if using) and cook for another 30 seconds.

Make the Sauce: Drain the soaked cashews and rinse them. Carefully transfer them to a high-speed blender. Add the nutritional yeast, lemon juice, salt, and black pepper. Pour in 1/2 cup of the reserved hot pasta water and blend on high until the sauce is completely smooth and creamy.

Combine and Finish: Add the sauce to the skillet with the aromatics. Add the fresh spinach and stir until it has wilted. Add the cooked and drained pasta to the skillet. Toss to coat everything evenly, adding a little more pasta water as needed to create a smooth, clingy sauce.