Why this recipe works:
Tasters preferred gruyère—a Swiss cow’s milk cheese with a nutty flavor—in our Quick Croque Monsieur. However, we found we could use deli Swiss if gruyère was unavailable. Instead of cooking one or two sandwiches at a time in a skillet, we used a rimmed baking sheet and a hot oven to cook four sandwiches simultaneously.
| 3 | tablespoons softened butter |
| 2 | tablespoons all-purpose flour |
| 1 | cup whole milk |
| 2½ | cups shredded Gruyère cheese |
| Salt and pepper | |
| 8 | slices hearty white sandwich bread |
| 2 | tablespoons Dijon mustard |
| 6 | ounces thinly sliced deli ham |
1. Adjust oven rack to upper-middle position and heat oven to 475 degrees. Melt 2 tablespoons butter in saucepan over medium heat. Whisk in flour and cook until golden, about 2 minutes. Slowly whisk in milk and simmer until slightly thickened, 3 to 5 minutes. Off heat, stir in ½ cup cheese, ¼ teaspoon salt, and ¼ teaspoon pepper. Cover and keep warm.
2. Spread remaining butter over 4 slices of bread. Arrange, buttered-side down, on rimmed baking sheet. Spread each slice with mustard, then divide ham, 1 cup cheese, and ½ cup sauce over mustard-topped slices and top with remaining bread. Top with remaining sauce and cheese. Bake until cheese is golden brown and bubbling, 8 to 10 minutes. Serve.