Oysters are best served very cold and this dressing is a superb compliment to the mineral saltiness of oysters.
Slice the tomatoes in half and marinate with the thyme, sliced clove of garlic and salt and pepper. Leave to marinate for 1 hour.
purée the marinated tomatoes and pass through a fine sieve, retaining the juice.
Add the remaining seasoning and finally whisk in the oil.
Chill over ice until very cold.
Shuck the oysters (open with a small, thick-bladed knife), or get the fishmonger to shuck them and loosen the meat from the shell.
Serve over crushed ice and salt, dressed with the cold dressing and finely grated fresh horseradish.