In a 2-inch high pan, arrange the sardines in a single layer, flesh side up. Sprinkle the fish with the salt and sugar. Drizzle the fish with the lemon and orange juices and vinegar. Scatter the zest, thyme, and pepper over the sardines. Cover the fish with 1 cup of olive oil, and then cover the pan with plastic wrap. Refrigerate overnight until the sardines are fully cured. Remove the cured sardines from the liquid and cover with additional olive oil to preserve.