Cut 1 chicory into fine long strips and rinse under cold water, so they don't discolor. Cut the 3 chicory heads very finely and sauté them in the butter, let them color further, stir occasionally and cook until nicely caramelized.
Meanwhile, spread 1 side of the 8 slices of bread with butter, turn them over on a board and generously spread the other sides with cheese sauce. Sprinkle 4 slices with a little grated cheese, and 2 slices with ham. Distribute the caramelized chicory and the raw chicory on top, placing the slices of bread with cheese sauce facing down on top.
Place in a frying pan and let cook gently for 3 to 4 minutes until the bread is nicely toasted and colored. Turn them over and cook the other side in the same way. Now distribute the rest of the cheese on top of the 4 sandwiches and gratin for 4 to 5 minutes in the oven on grill mode.
Tips
You can use any ground cheese except mozzarella, is a bit too faint in taste, but therefore Emmental Cheese, Comté, Gouda cheese or goat cheese. If you want it more adventurous, blue cheese can also be!