Use whatever combination of mushrooms strikes your fancy (I used a mix of shiitake and oyster). If you have
already-caramelized onions, substitute a heaping 1/2 cup. Once cooled, store the muffins in an airtight container; they can be refrigerated for up to a week, or frozen for up to a month. To reheat, warm through in a 350°F oven or toaster oven — it'll take a few minutes — (if frozen, thaw overnight in the fridge before reheating).