Wash, dry and slice the tomatoes. Wash the salad, dry it and cut it into strips. Cut the buns in half. Turn on the oven at 160° and line a baking tray with baking paper. Remove the rind from the Fontina and cut 8 slices of about 25 g each, put them on the baking tray and put them in the oven while cooking the meat.
Heat 4 tablespoons of oil in a large non-stick pan and cook the burgers 4 minutes per side, turning every 2 minutes. Remove them from the pan and place them on kitchen paper.
In the pan where you cooked the meat, place the buns with the cut side down and brown them for 30-40 seconds.
Coat the two cut sides of the buns with a thin layer of mustard and stuff them with 2 slices of tomato, the salad, a slice of Fontina, a burger, another slice of Fontina, more tomatoes and salad. Close them by pressing them lightly and serve immediately.