Finely chop garlic and red onion. Mix crème fraîche with lemon juice, salt, sugar and pepper.
Add garlic, red onion and thyme, and taste with salt and maybe a little salt and pepper.
Remove the herring from the jar and add to the new sauce.
Pour this over into a jar or glass and cover with a lid.
The herring should set in the refrigerator for 24 hours before eaten.
The herring can be kept in the refrigerator for approx. 1 week.