How to Make It
Preheat the oven to 400°. In a small saucepan, melt the butter over moderately high heat. Stir in the flour, then whisk in the milk until smooth. Bring to a simmer, whisking. Stir in the onion and bay leaf and simmer over low heat, whisking, until no floury taste remains, 10 minutes. Pass through a coarse strainer set over a small bowl; discard the bay leaf. Stir in the horseradish and season with salt and pepper. Press a piece of plastic wrap onto the sauce.
In a small bowl, combine the panko and cheese and stir in the olive oil. Arrange the phyllo shells on a rimmed baking sheet. Place an oyster in each shell. Spoon about 1 teaspoon of the horseradish sauce into each shell, and sprinkle the panko mixture on top. Bake in the upper third of the oven for about 10 minutes, until hot throughout and crisp on top. Serve right away.