Preheat the oven to 375°F.
Line a cookie sheet with parchment paper.
Remove the stems from the mushrooms and put the mushroom caps on the cookie sheet.
Put ⅓ cup of cream cheese, ¼ cup grated Parmesan cheese, ½ cup shredded cheese, ½ teaspoon garlic powder, and ½ teaspoon onion powder in a medium mixing bowl.
Chop the pre-cooked shrimp into small pieces.
Chop all the spinach and chives, making sure to remove the stems from the spinach.
Add the chopped shrimp and spinach to the bowl with the mixed cheeses and seasonings and then stir these ingredients to combine them.
Put a spoonful of the filling into each mushroom cap.
Put the seasoned croutons in a plastic bag and use a rolling pin to crush the croutons.
Sprinkle the crushed croutons on top of the filled mushroom caps.
Bake for 15 minutes or until golden brown.
When done, remove the shrimp stuffed mushrooms from the oven and allow to cool for 5 minutes and then serve.