- Melt 4 tablespoons butter in a large saucepan over medium heat. Whisk in the flour to make a roux, stirring frequently for about 5 minutes, until it turns blonde. Add the leeks, garlic, parsnips, and fennel, reduce the heat to low, and cook for another 5 minutes. Slowly add the milk while whisking. Increase heat, bringing to a boil. Add the bay leaf and thyme then reduce heat to low and simmer, stirring occasionally for 15 minutes.
- Remove the pan from the heat, remove the bay leaf, and season with the salt and pepper. Add the truffle oil and let cool to room temperature, about 35 minutes.
- Preheat the oven to 400 degrees. Place the oysters in a baking dish in a single layer, then pour the cooled sauce over the top of the oysters.
- In a mixing bowl stir together the breadcrumbs, 4 tablespoons melted butter, olive oil, Parmigiano-Reggiano, parsley, and tarragon. Sprinkle over the oyster and sauce in the baking dish. Bake for 15−20 minutes, until the sauce bubbles around the edges and breadcrumbs are golden brown.