In an electric spice grinder or high-power blender, combine nutritional yeast, dried mushrooms, lemon zest, garlic powder, and thyme and blend until a fine powder forms.
Place popped popcorn in a large mixing bowl and drizzle melted butter all over, tossing to coat evenly. Add about ¼ cup seasoning mixture, tossing to coat evenly; add more, if necessary, until popcorn is well coated (leftover seasoning mixture can be refrigerated in an airtight container for up to 5 days). Season with salt and serve. Popcorn can be stored at room temperature in a zipper-lock bag overnight.
