1teaspoonDiamond Crystal kosher salt(or 1/2 teaspoon fine sea salt)
Preheat oven to 400 degrees F. Spray a 2-quart baking dish with olive oil.
Place the broccoli florets in a large microwave-safe bowl. Add 2 tablespoons water. Cover and microwave on high until tender-crisp, about 3 minutes.
Meanwhile, in another large bowl, prepare the creamy sauce. Mix together the mayonnaise (or sour cream), Dijon mustard, kosher salt, black pepper, garlic powder, cayenne pepper and half of the grated cheddar cheese. Add a little water to thin out the mixture, about 1 tablespoon.
Drain the broccoli well and add to the creamy sauce. Gently toss to coat.
Transfer the coated broccoli to the prepared baking dish. Lightly spray the top with olive oil. Evenly sprinkle the remaining cheddar on top.
Bake the broccoli casserole until golden brown and bubbly around the edges, 15-20 minutes. If your baking dish is broiler safe, you can simply broil the broccoli casserole in the middle of the oven until browned, 2-3 minutes. This is actually a better method because it ensures the broccoli doesn't become overcooked.