1. Preheat the oven to 350 degrees F (or 180C) twice. Once for toasting the bread slices and next for baking the croque monsieur at 350 degrees.
2. In a large baking sheet, place the bread slices in layers and bake it in the center rack of a preheated oven for 20 minutes, turning the slices over halfway through.
3. Grease a 9x13-inch glass baking dish with oil. Keep aside.
4. In a large bowl, whisk the eggs with the Dijon mustard. Whisk in the milk and salt and keep aside.
5. Place 6 slices of the toasted bread in a single layer on the bottom of the baking dish. Pour one-third of the egg mixture evenly over the top. Sprinkle 1 cup cheese, spread the ham slices over the cheese and again sprinkle with 1 cup of cheese. 6. Place the remaining 6 bread slices over the cheese and press down firmly. Pour the remaining egg mixture evenly over the top and repeat with sprinkle the remaining 1 cup of cheese. Cover the dish and refrigerate at least 8 hours or overnight.
7. When ready to bake the croque-monsieur, place the dish in the bottom third of a preheated oven. Bake the dish, uncovered, for 45 minutes or until the top is golden brown.
8. Cut and serve the croque-monsieur with a bowl of asparagus-spinach salad.