Preheat your oven to 180°C (350°F). Prepare a 9x13 inch baking dish by lightly buttering or spraying it to prevent sticking.
Chop the broccoli into bite-sized pieces, then steam or blanch for 2-3 minutes until just tender and vibrant green. Drain thoroughly to remove excess moisture.
In a large mixing bowl, combine the cooked broccoli, shredded cheddar, cream of mushroom soup, chopped onion, and a pinch of salt and pepper. Stir everything together until well coated and evenly mixed.
Melt the butter and pour it into a small bowl. Add the panko breadcrumbs, then toss until the crumbs are coated and slightly clumpy.
Transfer the broccoli mixture into the prepared baking dish, spreading it out evenly with a spatula. Smooth the surface for an even bake.
Sprinkle the buttered panko breadcrumbs evenly over the top of the casserole, creating a golden, crispy crust.
Bake uncovered in the preheated oven for 25-30 minutes, until the top is crispy and golden brown, and the filling is bubbling around the edges.
Remove the casserole from the oven and let it rest for about 5 minutes. This helps it set and makes slicing easier.
Slice into portions, serve warm, and enjoy the cheesy, crispy goodness with your favorite side.