Preheat your oven to 375°F (190°C) and grease a 9x13 inch baking dish.
Blanch the broccoli in boiling water for 3-4 minutes until bright green and barely tender. Immediately transfer to an ice bath to stop the cooking process.
In a large saucepan, melt 2 tablespoons of butter over medium heat. Add onions and cook until translucent (about 5 minutes), then add garlic and cook for another minute.
Sprinkle flour over the butter mixture and whisk constantly for 1 minute to create a roux.
Gradually pour in milk and cream while whisking continuously to prevent lumps. Cook until the sauce thickens, about 3-4 minutes.
Remove from heat and stir in 2 cups of cheddar cheese until melted. Season with salt, pepper, and paprika.
Combine the sauce with the drained broccoli and transfer to the prepared baking dish.
Mix remaining cheese with panko breadcrumbs and melted butter, then sprinkle over the casserole.
Bake for 25-30 minutes until golden brown and bubbly.