Preheat the oven to 400°F (200°C). Spray a 2-quart baking dish with olive oil.

Place the broccoli florets in a large microwave-safe bowl and add 2 tablespoons of water. Cover and microwave on high until tender-crisp, about 3 minutes.

In another large bowl, mix together the mayonnaise, Dijon mustard, kosher salt, black pepper, garlic powder, cayenne pepper, and half of the grated cheddar cheese. Add about 1 tablespoon of water to thin the mixture if necessary.

Drain the broccoli well and gently toss it with the creamy sauce to coat.

Transfer the coated broccoli to the prepared baking dish. Lightly spray the top with olive oil and evenly sprinkle the remaining cheddar on top.

Bake the casserole until golden brown and bubbly around the edges, about 15-20 minutes.

Let it rest for 5 minutes before serving.