Soak cashews in water overnight, chilled.
Drain and add to a blender with. lemon juice, nutritional yeast, 3/4 cup cold water, and the fine sea salt. Purée until smooth, at least 3 minutes, adding more water, 1 tbsp. at a time, until sauce is as thick and smooth as sour cream but not as thick as hummus.
Taste and season with more salt or lemon juice; sauce should be tangy but not sour. Chill at least 30 minutes before serving.