1) Preheat a sandwich toaster or panini press.
2) Place 1 to 2 slices of fontina on top of one slice of bread. Top with the onion slices and season with salt and pepper. Add the spinach and prosciutto and the remaining slice(s) of fontina.
3) Top the sandwich with the second slice of bread slice. Brush both sides of the sandwich with the oil.
4) Toast the sandwich until the bread is golden brown and the cheese has begun to melt, about 2 to 4 minutes.