In a medium saucepan, add potatoes and water to cover; bring to a boil over medium-high heat. Cook until fork tender, 8 to 10 minutes. Drain, and set aside.
In a small Dutch oven, heat milk, stock, and Pernod over medium heat until steaming. Add oysters and reserved oyster liquor; cook until edges of oysters begin to curl, 4 to 5 minutes. Remove oysters from liquid, and set aside; reserve poaching liquid in Dutch oven.
In a small skillet, melt butter over medium heat. Add shallot, garlic, kosher salt, and pepper; sauté until fragrant. Add flour; cook, whisking constantly, until incorporated and bubbly, 1 to 2 minutes. Add shallot mixture to poaching liquid, whisking to combine. Add potatoes, Worcestershire, and celery salt; using an immersion blender, blend until smooth. Reduce heat to low; add oysters, and spoon into bowls. Garnish with hot sauce and thyme, if desired.
*Pernod Ricard is an anise-flavored liqueur. Herbsaint or absinthe can be substituted.