How to Make It
Preheat the oven to 350°. In a small baking dish, pour the olive oil over the onion wedges, cover with foil and roast for about 40 minutes, until the onion is tender. Drain, reserving the oil for another use. Coarsely chop the onion.
In a large, deep pot, combine 1 cup of water with the clams, white wine vinegar and foie gras mousse. Cover and steam over moderately high heat until the clams open, about 10 minutes. Transfer the clams to bowls, discarding any that do not open. Strain the broth into a medium bowl and rinse out the pot.
Return the sauce to the pot, stopping when you reach the grit at the bottom. Add the butter and whisk over moderate heat until creamy. Add the roasted onion and cook just until heated through. Pour the sauce over the clams and serve right away with crusty grilled bread.