Croque Monsieur Pie

Name: Croque Monsieur Pie
Ingredients: 3 tablespoons butter, plus more for greasing; 12 thin slices stale or fresh white bread, crusts trimmed (1/2- to 3/4-inch thick); 12 thin slices mild ham; 2 round tablespoons flour; 2 cups whole milk; Freshly grated nutmeg; Salt and pepper; 2 tablespoons Dijon mustard, plus more for serving; 3 large eggs; 8 ounces gruyere or Swiss cheese, shredded (about 2 cups); Finely chopped fresh parsley, for garnish; Mixed greens dressed with lemon juice and vinegar, for serving