Cook the chorizo and garlic (optional) in a pan over medium heat, for 5-7 minutes, until fully cooked. Strain the chorizo over paper towels to remove the excess grease. Set aside.
Remove the stems from each mushroom and wipe any dirt away with a paper towel. Separate mushrooms into two groups, by size. This will ensure that the mushrooms get the proper amount of steaming time in the microwave before baking.
Arrange the smaller mushrooms on a plate, cap side down (gill side up), and cover with three layers of damp paper towels. Microwave for 2 minutes. Remove and place upside down to let any excess water drain off.
Arrange the large mushrooms on a plate, cap side down (gill side up), and cover with three layers of damp paper towels. Microwave for 3 minutes. Remove and place upside down to let any excess water drain off.
Add cream cheese and pecans to a bowl and mix until well combined. Add the strained chorizo and mix until well combined.
Take a bit of the cream cheese mixture and add it to each mushroom cap. I like to add a generous amount and stuff each mushroom by hand.
Bake for 10 minutes, middle rack of oven.
Remove from oven. Increase oven temperature to 400 degrees Fahrenheit. Add a pinch of shredded cheese to each stuffed mushroom.
Put back in the oven on the top rack for 3-4 minutes, until cheese has just melted. Let cool for 2 minutes.
If you happen to have any leftover cream cheese chorizo mixture, spread it on a cracker or tortilla chip as a quick snack while you wait for the stuffed mushrooms to be done.