Cut or tear the prosciutto into bite-size pieces. Place a large skillet over medium-high heat; add 1 tablespoon olive oil. Add the prosciutto in a single layer (you may need to do this in two batches); cook, turning occasionally, until crisp, about 1 minute per side. Drain on a paper-towel-lined plate.
Add the remaining 2 tablespoons olive oil to the skillet over medium-high heat. Whisk the vinegar, mustard, 1/2 teaspoon salt, and pepper to taste into the oil to make a smooth dressing. Add the peaches, plums and red onion to the skillet; toss to coat and wilt the onion slightly. Transfer the mixture to a large serving bowl.
Add the greens and half of the gorgonzola to the bowl; toss to coat with the dressing. Divide the salad among plates; top with the crisp prosciutto and the remaining gorgonzola. Serve with the bread.