Preheat the oven to 375°F. Set up an ice water bath.

Bring a large saucepan of water to a boil and add 1 tablespoon of salt. Blanch the pearl onions for 1 to 2 minutes until the skins loosen.

Using a slotted spoon, transfer the onions to the ice bath to cool. Drain the onions and pat dry. Trim off the root ends and pinch off the skins.

Add the peppercorns and bay leaves to the saucepan of water and bring to a boil. Add the peeled onions and simmer over moderately high heat until just tender, 5 to 8 minutes.

Drain well and pat dry. Transfer the onions to a 9- x 13-inch baking dish.

In a medium saucepan, melt the butter over moderately low heat. Add the flour and cook, stirring constantly with a wooden spoon, until bubbling and just beginning to turn color, about 3 minutes.

Gradually whisk in the milk and simmer, stirring, until the sauce is thick enough to coat the back of a spoon and no floury taste remains, about 5 minutes.

Stir in the sage and thyme and season generously with salt and pepper.

Pour the sauce over the onions and bake for 20 minutes, until bubbling and just starting to brown at the edge.

Let stand for 5 minutes before serving.