Step 1

Preheat the oven to 400°. Set a steamer basket in a large, deep saucepan filled with 1 inch of water and bring to a boil. Add all of the broccoli, cover and cook over high heat until crisp-tender, about 5 minutes. Transfer the broccoli to a paper towel–lined baking sheet to drain.

Step 2

In a medium saucepan, heat the olive oil. Add the leeks and a generous pinch of salt and cook over moderate heat until softened, about 10 minutes. Add the flour and whisk over moderately high heat until bubbling and a paste forms, about 2 minutes. Add the milk and cook, whisking constantly, until thickened and bubbling, about 5 minutes. Remove from the heat and whisk in the cheddar and cayenne. Season with salt and pepper.

Step 3

Pour half of the cheese sauce into a large 2-inch-deep baking dish. Arrange the drained broccoli on top of the cheese sauce in a single layer and drizzle the remaining cheese sauce on top. In a small bowl, mix the crackers with the almonds and melted butter and sprinkle over the broccoli. Bake the casserole for 15 minutes, until the sauce is bubbling and the top is golden brown.