To prepare the bread cases, preheat the oven to 180°C. Use a rolling pin to flatten each slice of bread to a thickness of 1–2 mm. Use a 7 cm round cutter to cut one round from each piece of bread. Lightly brush both sides of the bread with melted butter and then press into 24 small tartlet pans. Bake in the preheated oven for 10–12 minutes, or until light golden.
To make the sauce, combine the milk, bay leaf, onion, peppercorns and celery in a small heavy-based saucepan. Heat on medium heat until bubbles form around the edge of the saucepan. Remove from the heat and stand at room temperature for 15 minutes to cool. Strain and reserve the milk, discarding the contents of the sieve.
Melt the butter in a small saucepan until hot and bubbling. Add the flour and stir constantly for 1 minute. Remove from the heat and add the strained milk in a slow, steady stream, whisking constantly to avoid the sauce becoming lumpy. Return the saucepan to the heat and cook, stirring constantly on medium-high heat until the sauce comes to the boil and thickens. Stir in the prawns, or mushrooms, or asparagus. Season to taste with salt and white pepper.
Spoon about 1 tablespoon of the filling into each bread case and return to the oven to bake for a further 5 minutes, or until they are warmed through. Sprinkle with chopped chives and serve immediately.