Preheat a panini grill on the "Medium" setting.
While the grill is preheating, assemble the panini.
Place the Fontina or Swiss cheese on the bread, being sure to cover the surface, followed by a few leaves of arugula or spinach.
Next, add a layer of prosciutto and another layer of cheese followed by the remaining slice of bread.
Brush each side of bread with a light coating of olive oil.
When the panini grill "Ready" light is illuminated, place the panini on the grill, lower the top grill and toast for about 3 - 5 minutes (check the panini after 2 minutes for doneness, continue to toast if necessary)
Remove with a nylon spatula and serve warm.