Ham and Leek Sandwich with Cheddar Sauce Recipe

Ingredients (9)

• 3 tablespoons unsalted butter
• 2 medium leeks, white parts thinly sliced (about 1/2 cup)
• Kosher salt and freshly ground black pepper
• 3 ounces French-style cooked ham, finely sliced
• 1 tablespoon all-purpose flour
• 3/4 cup warm milk
• 2 ounces grated aged white cheddar or gruyere cheese (about 1/2 cup)
• 1 teaspoon Dijon mustard
• 1 large croissant or 1 (7-inch) piece baguette, split in half lengthwise