Season burgers with salt and pepper, and put on a hot grill. When the burgers are almost done, place one slice of fresh mozzarella on top of each burger, and melt the cheese just slightly. Pull the burgers off the heat and let them rest 5 minutes. To build the burger, spread basil pesto on both halves of the bun, place burger pattie on top, and then one big slice of tomato. Sprinkle with a little Fleur de sel, and place bun top, on top.