Place the cashews in a high-sided bowl. Bring 2 cups of water to boil, then pour enough water over the cashews to cover. Soak for 20–30 minutes, then drain and rinse the cashews.
Place soaked cashews, coconut milk, salt and pepper into a food processor. Process on high until mixture is smooth and creamy (about a minute). Add the nutritional yeast, then process on low for another 15 seconds.
Heat a large, high-sided skillet over medium-high heat. Add olive oil to the hot pan, along with shallots and garlic. Sauté the shallots and garlic until translucent and fragrant (about 2 minutes), stirring constantly to prevent burning. Reduce heat to medium and add the cashew mixture to the pan. Bring the mixture to a low simmer, whisking constantly. As the mixture thickens, add a bit of pasta water (a little at a time) and lemon juice (if using), until sauce reaches desired consistency.