Either buy sliced lardo from your nearest Italian specialty store, or to prepare your own, combine salt, rosemary, thyme, black pepper and cover the fatback. Leave covered & refrigerated in an airtight plastic container for four to six months until the fat is firm to the touch.
Combine caramel, red wine, red wine vinegar in a sauce pot and bring to a simmer. Add sliced onions and cook for 8 to 10 minutes until tender. Add balsamic vinegar and cook for 3 to 4 minutes until liquid is slightly reduced, and allow to cool.
Warm bread in oven until it is slightly toasted, add 1 tablespoon of onion mixture to each piece and cover with the thinnest slice of lardo possible. Place back in oven for 1 minute until lardo starts to shine.
Dress watercress with 3 tablespoons of the liquid from the onion mixture, olive oil and season with salt and pepper. Divide watercress between four plates, place three crostino on top of each salad and drizzle with Aceto Manadori.