A classic recipe for oysters, said to be "as rich as Rockefeller," gets an update from chef Alex Guarnaschelli, Food Network host and executive chef at The Darby.
Preheat the oven to 375 degrees.
Place a strainer over a medium bowl; gently shuck the oysters over the strainer, using the strainer to catch the meat and strain the liquor into the bowl. Reserve the deeper halves of each oyster shell (discarding remaining shells) and scrub clean under cold running water; dry shells and arrange in a single layer on a baking sheet; set aside.
In a medium skillet, melt butter over medium heat. Add shallots; season with salt and cool until tender, 3 to 5 minutes. Stir in flour. Add Herbsaint and cook until almost all the liquid has evaporated. Add garlic and anchovies and their oil; stir to combine. Season with salt. Add heavy cream and reserved oyster juices; let cook 1 minute. Stir in spinach, watercress, basil, and tarragon. Cook until mixture thickens, 3 to 5 minutes. If mixture seems too thick, add a little water or heavy cream to thin.
Place reserved oyster shells in oven and bake until warm, about 2 minutes. In a medium bowl, mix together cheese, panko, and breadcrumbs until well combined; set aside.
Spoon scant tablespoon spinach mixture into each of the shells and top each with a whole oyster. Top with a little more of the spinach mixture and sprinkle with the breadcrumb mixture. Transfer to oven and bake until oysters are warm to the touch, 2 to 3 minutes.
Preheat broiler. Place oysters under broiler to brown. Garnish with scallions and season with lemon juice; serve immediately.