Preheat your oven to 180°C (350°F) and lightly butter a casserole dish to prevent sticking.
Wash and chop the broccoli into small, bite-sized florets. Roast them on a baking sheet for about 10 minutes until slightly charred and fragrant, then set aside.
In a large mixing bowl, whisk together the cream of mushroom soup, sour cream, and shredded cheddar cheese until smooth and creamy. This forms the rich base of your casserole.
Add the roasted broccoli and diced onion to the cheese mixture, stirring gently to coat everything evenly. Season with salt and pepper to taste.
Pour the mixture into the prepared casserole dish, spreading it out evenly with a spatula.
In a small saucepan, melt the butter. Toss the panko breadcrumbs in the melted butter until they are fully coated, then sprinkle the breadcrumb mixture evenly over the casserole for a crispy topping.
Bake uncovered in the oven for 25-30 minutes until the casserole is bubbling around the edges and the breadcrumb topping is golden brown and crispy.
Remove from the oven and let it rest for 5 minutes to allow the casserole to set. The top should be crispy, the cheese gooey, and the broccoli vibrant green.
Serve hot, with extra cheese or herbs if desired, and enjoy the comforting, cheesy goodness in every bite.