In a medium saucepan, melt the butter over medium heat. Add the leeks, shallots, celery, and mushrooms. Sauté until tender but not colored. Add the wine and oyster liquor and reduce by half. Add the heavy cream and reduce until nappé, slightly thickened. Add the lemon juice, oysters, and tomatoes. Spoon the stew into center of soup plate. Arrange toasts around and garnish with chives and tarragon.