Remove Chorizo from the casing and saute in a medium frying pan for a few minutes. Add the chopped onions and cook for about 3 minutes until translucent.
Meanwhile, clean off the mushrooms and remove the stems. Place the caps on a large plate and microwave for 2 minutes to soften. Nothing worse than biting into a stuffed mushroom that is still raw. For me anyway.
Add the cream cheese, shredded cheese, and baby spinach leaves to the Chorizo mixture. Stir well and cook for a minute or two until the spinach has wilted and mixes in. Remove from the heat.
Stuff the mushrooms with about a Tbsp each of the filling.
Place on a cookie sheet and bake in a 375 degree (F) oven for about 10 minutes. Longer if you like your mushrooms really soft.
Remove and cool for a few minutes before eating or pay the consequences (like I did) with a blistered mouth for the next few days (it was worth it).