We went all out with our new Croque Monsieur; it's a triple cheese threat made with Fontina in the sandwich, Parmesan in the bechamel sauce, and grated Gruyere on top.
Preheat oven to 450 degrees with rack in bottom third. Blanch broccoli rabe in batches in a pot of salted boiling water until just tender, about 2 minutes. Drain.
Heat oil in a large skillet over medium heat. Add garlic; cook, stirring, 30 seconds. Add broccoli rabe; cook, stirring, 2 minutes. Season with salt. Let cool slightly, then finely chop.
For each sandwich, spread 2 teaspoons jam on one bread slice and 1 teaspoon mustard on another. Layer one slice with cheese, broccoli rabe, and ham, dividing evenly. Close sandwich; brush each side with butter.
Place the sandwiches on a baking sheet, top with 2 tablespoons bechamel and 1/4 cup grated Gruyere. Broil until golden brown and bubbling, about 10 minutes.
For a Croque Madame, top each sandwich with a fried egg.