Peel the onions, cut them in half, remove the root in the heart, then finely slice each half onion, from top to bottom.
Heat 3 cl of olive oil in a sauté pan. Sweat the onions without coloring for 5 minutes. Add the oregano, bay leaf, fleur de sel and freshly ground pepper. Cook covered for 30 minutes, stirring frequently. The onions must remain transparent.
Rinse the anchovy fillets under running water, pat dry. Add them to the onions, let them melt. Remove from fire. Remove the bay leaf.