Bake these ahead for a quick snack or to enjoy alongside breakfast.
Preheat oven to 350° F.
In a bowl, whisk together the flour, baking powder, baking soda, garlic and onion powders, and salt. Stir in both cheeses and the jalapeños if using.
In another bowl, whisk together the melted butter, egg and milk. Add the wet ingredients to the dry ingredients and stir in the chopped spinach. Mix until all ingredients are incorporated.
Butter or spray the insides of a muffin tin. Fill each one 3/4 of the way. Bake for 30 minutes. Cool baked muffins on a rack.