Season burgers with salt and pepper, and then grill on a hot grill. While the burgers are grilling, also place the slices of pineapple, and red onion on the grill, and grill them as well, to cook them slightly. When the burgers are almost done, place 1 slice of fontina cheese on each burger, and melt slightly. Remove burgers from heat and let them rest for five minutes. To build, place one slice of grilled red onion, on the bottom half of the bun, place burger on top with a slice of pineapple, and lettuce leaf on top, and the mango chutney on the top half of the bun.