Bring a large pot of water to a boil, then add the broccoli florets and blanch for about 2 minutes until bright green and slightly tender. Drain thoroughly and set aside.
In a skillet, melt 2 tablespoons of butter over medium heat. Add the diced onion and sauté until translucent and fragrant, about 5 minutes. Toss in the blanched broccoli and cook for another 2 minutes, letting the flavors meld. Remove from heat.
In a mixing bowl, stir together the cream of mushroom soup, shredded cheddar cheese, and sour cream until smooth and creamy.
Pour the broccoli mixture into a greased 9x13 inch baking dish, spreading it out evenly. Pour the cheese sauce over the top, making sure to cover all the broccoli.
In a small bowl, combine panko breadcrumbs with melted butter, mixing until the crumbs are coated and slightly golden. Sprinkle this evenly over the casserole for a crispy topping.
Bake the casserole in a preheated oven at 180°C (350°F) for 25-30 minutes, until bubbling around the edges and the breadcrumb topping is golden brown.
Remove the casserole from the oven and let it rest for 5 minutes to set. The cheese will be gooey and the top crispy, inviting you to dig in with a spoon.
