A classic croque monsieur recipe with all the richness and complexity of Dijon, Gruyere, and ham, plus a bit of spinach to balance out all that butter.
Heat an electric skillet to 350 degrees, or heat a large skillet over medium-high heat.
In a medium bowl, whisk together eggs, milk, and salt. Set aside.
Use a fork to whip together 2 tablespoons softened butter and 1 teaspoon Dijon mustard. Spread evenly on one side of each slice. For each sandwich, add half the cheese, 1/4 cup spinach, 1 slice of ham, and then the remaining cheese.
Melt remaining tablespoon butter in the griddle or skillet.
Dip each sandwich in egg mixture, turning to coat both sides.
Place both sandwiches in hot skillet. Cover, and cook 4 minutes. Flip, cover again, and cook until sides are golden brown and cheese is melted, 3 - 4 minutes.
Slice in half and serve immediately.