In a large skillet, cook the bacon over moderate heat, stirring occasionally, until crisp, about 8 minutes. Using a slotted spoon, transfer the bacon to paper towels to drain. Pour off all but 1 tablespoon of the bacon fat from the skillet and add the remaining 1 tablespoon of butter. Add the leeks, celery and onion and cook over low heat, stirring occasionally, until softened, about 10 minutes. Stir in the flour and cook for 1 minute. Slowly add the reserved oyster-poaching liquid, whisking constantly, until smooth. Simmer over low heat, whisking often, until no floury taste remains, about 3 minutes. Whisk in the milk, cream and thyme and simmer for 3 minutes longer, until thickened and creamy. Stir in the lemon juice, then season with salt, pepper and Tabasco. Fold in the oysters and cook just until warmed through, about 1 minute.