For the raw marinated tuna, first mix a marinade of white wine, sherry vinegar, lime juice, the whole thyme sprig, bay leaves, sea salt and pepper.
Place the tuna in a shallow dish, pour marinade over it and marinate for about 1 hour.
Remove from marinade and refrigerate. Strain the residue and set marinade aside.
Cut the onion into strips and saute with the garlic in olive oil until translucent. Deglaze with the strained marinade, add fish stock and reduce by half. Stir in the chopped capers and olives and let cool.
Arrange the tuna on plates, cover with the marinade and sprinkle with the chopped thyme.