Preheat your oven to 375°F (190°C) and grease a 9x13 inch casserole dish with olive oil.

Toss broccoli florets with olive oil, salt, and pepper, then roast on a baking sheet for 15 minutes.

Melt 4 tablespoons of unsalted butter in a large saucepan over medium heat, then sauté diced onion for 3-4 minutes until soft, followed by minced garlic for 30 seconds.

Sprinkle in the all-purpose flour and stir for 1-2 minutes to create a roux.

Gradually whisk in 2 cups of whole milk, continuing to stir until the mixture thickens, about 5 minutes.

Remove from heat and mix in 2 cups of shredded cheddar cheese, cayenne pepper, and Dijon mustard until melted.

Fold in the roasted broccoli and 1 cup of sour cream until evenly coated.

Pour the mixture into the greased casserole dish and top with crushed garlic butter pretzel pieces.

Bake for 20-25 minutes until the top is golden brown and edges bubble.

Let it rest for a few minutes before serving.