Broccoli and Cheddar Casserole with Leeks

Name: Broccoli and Cheddar Casserole with Leeks
Ingredients: 3 pounds broccoli, cut into small florets, stems peeled and sliced into 1/4 inch coins; 3 tablespoons extra-virgin olive oil; 2 leeks, halved lengthwise and thinly sliced crosswise; Kosher salt; 1/4 cup all-purpose flour; 3 cups whole milk; 6 ounces sharp white cheddar cheese, shredded (2 cups); 1/4 teaspoon cayenne pepper; Freshly ground pepper; 1/2 cup coarsely crushed saltine crackers; 1/2 cup sliced almonds; 4 tablespoons unsalted butter, melted