Heat the oven to 350 degrees F. Line a standard-size 12-cup muffin tin with paper liners.
Crumble the sausage or chorizo and put in a non-Teflon frying pan (I use a cast iron pan). Cook over medium-high heat, stirring frequently, until browned, about 8-10 minutes. Set aside.
With a sharp knife, poke small holes in the bag of spinach, put in a microwavable bowl, and place in the microwave on high for 3 minutes.
Cut a tiny edge off the corner of the bag, and squeeze as much water out of the bag as possible.
Place the drained spinach, eggs, olive oil, garlic, Italian seasoning, onion, salt and pepper in a high speed blender and pulse/blend for about 1 minute, or until thoroughly mixed. Transfer to a large bowl and stir in the sausage until well mixed.
Fill the muffin tins to just beneath the rim. Bake for 30-35 minutes, until the tops start to brown. Remove from oven and let cool before removing individual muffins from the liner.