Combine the peaches, fennel and onions in a medium bowl. Drizzle with olive oil and the lemon juice. Season with salt and pepper and toss with your hands.
Lay the slices of bresola on the edges of your serving platter and add the arugula to the center. Top the arugula with the peach-fennel mixture. Drizzle with more olive oil.
Serve the salad immediately after plating. It can't sit too long or the lemon juice will "cook" the bresola.