Bring a large pot of salted water to a boil, then add the broccoli florets. Blanch for 2-3 minutes until bright green and slightly tender, then drain well and set aside.
In a skillet over medium heat, melt the butter, then add the finely chopped onion. Sauté until translucent and fragrant, about 3-4 minutes, stirring occasionally.
Add the minced garlic to the skillet and cook for another 30 seconds until aromatic, being careful not to burn it.
In a mixing bowl, whisk together the heavy cream, grated cheddar, and Parmesan until smooth and creamy. Season with salt, pepper, and a pinch of smoked paprika.
Pour the cheese mixture into the skillet with onions and garlic, stirring constantly over low heat until the cheese melts and forms a smooth sauce.
Spread the blanched broccoli evenly into the prepared baking dish, then pour the cheese sauce over, ensuring all pieces are coated.
In a small bowl, mix the panko breadcrumbs with the melted butter, smoked paprika, and a pinch of salt. Toss until evenly coated.
Sprinkle the breadcrumb mixture evenly over the assembled casserole, covering the top completely.
Place the casserole in the oven preheated to 180°C (350°F) and bake for 25-30 minutes, until the top is golden brown and bubbling around the edges.
Remove from the oven and let it rest for 5-10 minutes to set and cool slightly, making it easier to serve slices.
Serve the casserole hot, garnished with fresh herbs if desired, and enjoy the crispy top contrasting with the creamy, cheesy interior.
