Homemade Broccoli Cheese Casserole

2 pounds broccoli
Kosher or sea salt, to taste
3 tablespoons unsalted butter, divided, plus more for greasing the dish
1/2 cup panko breadcrumbs
2 tablespoons all-purpose flour
1 cup whole milk
2 cups (8 ounces) shredded sharp cheddar cheese, divided
Freshly ground black pepper, to taste


Cook the broccoli:
Cut the heads of broccoli into florets (about 10 cups). Trim the dry ends off the stems and remove the outer layer using a peeler. Cut into coins.
Bring a medium to large pot of water to a boil. Add a generous pinch of salt. Add the broccoli stems and cook until slightly tender, about 3 minutes. Add the florets and cook until bright green and just tender enough that a fork can pierce the florets, 3 to 4 minutes.

Drain, add a few ice cubes on top, and rinse with cold water for about 30 seconds to stop the cooking. Line a sheet pan with a clean tea towel, dump the cooled broccoli on top, and let drain while you make the sauce. Gently rinse and dry the pot.

Preheat the oven to 400°F.
Butter a 9x13-inch glass casserole dish.

Toast the breadcrumbs:
In the same pot over medium heat, melt 1 tablespoon of butter. When the butter starts foaming, add the breadcrumbs and toss to coat in butter. Cook, stirring often, until the breadcrumbs begin to turn golden brown, 2 to 3 minutes. Scrape into a bowl and set aside. Carefully wipe out the pot and return it to the stove.

Make the sauce:
Add the remaining 2 tablespoons of butter to the same pot and melt over medium-low heat. Once melted, add the flour and whisk until a smooth paste forms. Cook until the roux takes on some color and begins to smell nutty, about 1 minute.
Slowly whisk in the milk and bring to a simmer. Whisk again to ensure there are no lumps, then switch to a wooden spoon or rubber spatula and stir often. Cook until the milk mixture has thickened and can coat the back of a spoon, 2 to 3 minutes. Remove from the heat and stir in 1 cup of cheese. Stir until smooth.

Assemble the casserole:
Add the drained broccoli to the prepared casserole dish, patting it into an even layer. Generously season the broccoli with salt and pepper, then drizzle the cheese sauce on top. Scatter the remaining shredded cheese all over, then top with the toasted breadcrumbs.

Bake:
Bake until the cheese is melted and the top of the casserole is nicely golden brown, about 20 minutes. Let sit for 5 to 10 minutes before serving.

Let leftovers cool completely before placing them in the fridge. Tightly sealed, they will last 2 to 3 days.