How to Make It
In a large saucepan, heat the oil. Add the onion and minced garlic; cook over moderate heat until softened, 5 minutes. Add the vermouth; boil over moderately high heat until reduced by half, about 2 minutes. Add the oyster liquor and water and simmer for 3 minutes. Add the cream and simmer over moderate heat until the sauce coats a spoon, 5 minutes. Remove from the heat. Add 1 teaspoon of the lemon juice; season with salt.
In a bowl, mix the butter with the tarragon and the remaining 1/2 teaspoon of lemon juice; season with salt. Lightly rub the toasts with the garlic clove and spread with the tarragon butter.
Add the oysters to the sauce. Cook over moderately low heat until warmed through, 2 minutes. Serve with the toasts.