How to Make It
Place a fine mesh strainer over a medium bowl, and drain oysters, reserving 1/3 cup oyster liquor.
Place potatoes and 1 teaspoon of the salt in a pot of water over high, and bring to a boil. Add collard stems. Boil 1 minute. Drain in a colander, and rinse under cold water.
Warm milk and cream in a medium saucepan over medium; cook until just beginning to boil. Remove from heat, and cover with a lid to keep warm.
Melt butter in a large Dutch oven over medium until foamy. Add onion, celery, potatoes, collard stems, and 1 teaspoon of the salt. Cook, stirring often, until onion is tender and translucent, 6 to 8 minutes. Sprinkle flour over vegetable mixture, and cook, stirring often, until thickened, about 2 minutes.
Slowly whisk in warm milk and heavy cream. Bring mixture to a boil, stirring often to keep mixture from sticking. Stir reserved oyster liquor into Dutch oven, and cook until all vegetables are tender, about 2 minutes.
Warm reserved bacon drippings in a large skillet over medium. Add drained oysters in a single layer. Sprinkle with a few grinds of black pepper and remaining 1 teaspoon salt. Cook just until oysters begin to curl around the edges. Immediately transfer oysters to Dutch oven; stir to combine. Remove from heat. Cover; let stand 10 minutes.
When ready to serve, ladle hot stew into bowls, and sprinkle each evenly with cayenne and a few grinds of black pepper. Top with cooked bacon, parsley, and celery leaves.