Preheat your oven to 375°F (190°C).

Bring a large pot of salted water to a boil. Add broccoli and cook for 2-3 minutes until just tender. Drain and set aside.

In a skillet, heat olive oil over medium heat. Sauté onion for 4–5 minutes until soft, then add garlic and cook for 1 more minute.

In a saucepan, melt butter over medium heat. Whisk in flour and cook for 1-2 minutes to form a roux. Slowly whisk in milk, stirring until smooth and thickened (about 3–5 minutes).

Remove from heat and stir in cheddar, Gruyère, salt, pepper, Dijon mustard, and sour cream until melted and well combined.

In a large mixing bowl, combine broccoli, sautéed onions, and cheese sauce. Stir well to coat.

Transfer mixture to a greased 9×13-inch baking dish.

In a small bowl, mix panko breadcrumbs with melted butter. Sprinkle evenly over the casserole.

Bake uncovered for 25–30 minutes, or until bubbly and golden on top.

Let rest for 5–10 minutes before serving.