Clean the sardines by cutting the heads and tails off with a scissors. Then, cut down the backbone to open the fish. Do not slice completely through. Remove the bone.
Coat the insides of the sardines with a covering of gray salt and close the fillet. Coat the outside with salt, and place in a shallow, small plate. Cover and place in the refrigerator for 12 hours.
Rinse well and slice in half to separate the fillets.
In a small bowl, whisk the lemon juice, olive oil, and oregano together. On a platter, lay the tomatoes out and place the sardine fillets on top. Pour the lemon mixture over the fish. Toss olives on top and serve with the rustic bread for meze.