Put the flour in a small bowl and line a plate with paper towels. Arrange them both near your cooktop.

Heat about ½ inch of olive oil in a medium skillet or wide saucepan over medium-high heat.

Season the fish all over with the salt and pepper. Dredge a piece of fish through the flour and shake off any excess. Test the oil temperature by dipping a corner of the flour-coated fish into the oil — if it sizzles vigorously, the oil is ready; if not, heat the oil for a bit longer. Carefully place the piece of fish in the oil and repeat the flouring process with another 4 to 6 pieces of fish; don't crowd the pan. Fry the fish, turning once, until cooked through and golden brown, about 2 minutes on each side. Transfer the fish to the paper towel-lined plate to drain. Continue frying the rest of the fish in batches.

Put the vinegar, olive oil, onion, and chopped cilantro in a deep bowl or other container and stir to combine well. Add the pieces of fried fish to the bowl, making sure they are all submerged in the liquid. Cover the bowl with plastic wrap and set aside for 2 hours at room temperature to marinate, or transfer the fish to the refrigerator to marinate for up to 8 hours.

Serve the escabeche at room temperature. Store the escabeche in an airtight container in the refrigerator for 3-4 days.