Preparation 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.
Preparation 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.