Preheat your oven to 375°F (190°C) and grease a 9x13 inch casserole dish with butter or cooking spray.

Toss broccoli florets in a bowl with olive oil, salt, and pepper. Spread on a baking sheet and roast for about 15 minutes.

In a medium saucepan over medium heat, melt the unsalted butter. Add diced onion and sauté for 3-4 minutes until softened.

Stir in minced garlic and cook for about 30 seconds until fragrant.

Whisk in all-purpose flour and cook for 1-2 minutes to form a roux.

Gradually pour in whole milk while whisking to prevent lumps. Heat until thickened, about 3-5 minutes.

Remove from heat and stir in shredded cheddar cheese, cayenne pepper (if using), and Dijon mustard.

Fold in roasted broccoli and sour cream until well coated.

Pour the mixture into the prepared casserole dish, spreading evenly. Top with crushed pretzel pieces.

Bake for 20-25 minutes, or until the top is golden brown and bubbling.

Let the casserole rest for about 5-10 minutes before serving.