Cut the stems off the watercress and chop the leaves into ½” pieces. Cut the stems off the parsley leave and mince with the celery leaves and stalk. Mince the tarragon.
Mince the garlic and finely chop the green onions.
Melt the butter in a large sauté pan over medium heat. Add the green onions and the garlic and cook for about 2 minutes until soft. Add the watercress, parsley, celery leaves and tarragon and cook until the greens are completely wilted, about 2 minutes.
Stir in the Pernod, Tabasco, and Worcestershire sauce. Stir in the bread crumbs.
Turn off the heat and cool for 10 minutes. The filling can be refrigerated at this point for later use. Store in an airtight container in the refrigerator for up to 4 days. If chilled, allow to soften slightly before using.
To cook, turn on the broiler or oven to high and put the rack about 4” from the heat source. Using an oven safe pan or dish, line the bottom with rock salt (this will keep the oysters stable and upright).
Shuck the oysters, taking care to not spill their liquor. Set them upright on the rock salt in the pan. Place about 1 tablespoon of filling on top of each oyster and set the pan under the broiler. Cook for about 6 minutes or until the top is browned and the oyster is cooked through, but not dry.
Serve on a plate on a bed of rock salt