Pre-heat a panini press or grill pan over medium heat.
Slice open each piece of ciabatta. Place a couple slices or a small handful of Fontina, four slices prosciutto and two leaves of sage into each sandwich. Place each sandwich into the pan and press with a foil-wrapped garden brick, a pan weighted with cans or a tea kettle filled with water. Brown and crust the sandwiches to melt the cheese, about 2-3 minutes per side.
While the sandwiches are pressing, place the arugula in a large salad bowl. Season the salad with the lemon juice, salt, freshly ground black pepper and EVOO, tossing to evenly coat the greens.
Serve the sandwiches with the arugula salad alongside.