Maybe your new favorite bean dish:
Drain beans. Cut onion into rings. Finely chop the garlic clove.
Stir vinegar, oil, pepper, salt, thyme and garlic into a marinade, stir in beans and onion.
Rinse lollo rosso, shred. Divide mackerel fillets into bite-sized pieces. Fold lettuce into beans, arrange and top with fish pieces.