For a spicier salsa, leave the seeds in the jalapeno pepper, or add a pinch of cayenne pepper. If you don't have fresh tomatillos, canned tomatillos can be used, but fresh are preferred. The salsa can be made ahead of time and stored in the refrigerator for up to 24 hours. This allows the flavors to meld together beautifully. Feel free to add other toppings to your tacos, such as shredded cabbage, avocado slices, or a dollop of sour cream or Greek yogurt.