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.