1 garlic clove
2 cups loosely packed fresh spinach
1 bunch watercress, stems trimmed
1/2 cup chopped green onions
3/4 cup (11/2 sticks) unsalted butter, room temperature
1/2 cup dry breadcrumbs
2 tablespoons Pernod or other anise-flavored liqueur
1 teaspoon fennel seeds, ground
1 teaspoon hot pepper sauce
1 pound (about) rock salt
24 fresh oysters, shucked, shells reserved
1/4 cup freshly grated Parmesan cheese
Position rack in top third of oven and preheat to 450°F. Finely chop garlic in processor. Add spinach, watercress and green onions to garlic. Process, using on/off turns, until mixture is finely chopped. Transfer mixture to medium bowl.
Combine butter, breadcrumbs, Pernod, fennel and hot sauce in processor. Process until well blended. Return spinach mixture to processor. Process, using on/off turns, just until mixtures are blended. Season with salt and pepper. (Can be made 8 hours ahead. Cover; chill.)
Sprinkle rock salt over large baking sheet to depth of 1/2 inch. Arrange oysters in half shells atop rock salt. Top each oyster with 1 tablespoon spinach mixture. Sprinkle with cheese. Bake until spinach mixture browns on top, about 8 minutes.