Slice rolls in half lengthwise. Mix mayonnaise into pesto. Spread the pesto mayo on both sides of all 4 sandwiches. Pit peaches and slice thinly. On each sandwich, layer fontina cheese, peaches, 2-3 slices prosciutto, and arugula. Make sure the fontina in next to pesto on both sides of sandwich for even melting. Put sandwich together. Heat grill pan over medium heat and brush olive oil on both sides of bread. Place on grill and weigh down with a pan with cans of beans, you are looking for enough pressure to create a pressed warm sandwich. Cook 3-4 minutes per side.