Set oven to 400F.
Chop up the shrimp into small pieces.
Set a nonstick skillet on medium heat. Once hot, spray with olive oil, then add the red onion and garlic. Sauté for 3 to 5 minutes, or until the onion has turned brown and translucent. Set aside.
Remove the stem from the mushroom caps and scrape out the inner gills using a spoon.
In a large bowl, combine all the ingredients for the stuffing.
Fold and mix everything together.
Evenly divide the stuffing among the mushroom caps.
Bake for 20 minutes, or until the shrimp turns white, pink and plump, and the outside is golden.