For a spicier muffin, use hot green chiles or add a pinch of cayenne pepper to the batter. Feel free to experiment with different cheeses, such as Monterey Jack or pepper Jack, for a unique flavor. These muffins are best served warm. They can be stored in an airtight container at room temperature for up to 2 days, or in the refrigerator for up to 5 days. For extra flavor, brown the onion in a pan before adding it to the other ingredients.