Preheat oven to broil. Line a small baking sheet with parchment paper.
In the work bowl of a food processor, place 4 baguette slices, celery, onion, spinach, butter, Pernod, garlic, thyme, salt, and pepper; pulse until combined.
Arrange remaining 12 baguette slices in a single layer on prepared pan. Place one oyster on each baguette slice, and top with vegetable mixture.
Broil until golden brown, about 4 minutes. Garnish with Parmesan and thyme, if desired.
Recipe by Southern Lady Magazine at https://southernladymagazine.com/oysters-rockefeller-crostini-recipe/