Place the fish in a non-reactive bowl and sprinkle liberally with vinegar. Let stand 2-3 minutes for anchovies, a bit longer for the sardines.  

Pinch the head off, not detaching it completely, and move your finger down the belly detaching the backbone, to discard it together with the intestines.  Snap the backbone at the tail, and the fish will open as a book. Rinse under cold water, and place each anchovy (or sardine) on thick paper towels, to drain completely.

Lay the  fish fillets in one layer, open side up, on a deep, non-reactive serving platter, preferably square or rectangular, add salt, splash with some lemon juice, sprinkle with garlic, the fennel, and the hot peppers, if using. Top with another layer of sardines, open side down, add salt, splash with lemon juice, sprinkle with garlic and the fennel, and continue with the rest of the fillets.

Pour olive oil over the anchovies or sardines to cover completely, and press the fish, so that the oil penetrates all layers.

Cover and refrigerate for 3-5 hours, and up to 6-7 days. The oil with solidify. Let come to room temperature for a few minutes, before serving.