Bring a large pot of salted water to a boil, then add the broccoli florets. Blanch for 2-3 minutes until bright green and just tender, then drain and set aside.
Preheat your oven to 375°F (190°C). Lightly grease the baking dish with butter or non-stick spray.
In a skillet over medium heat, melt 2 tablespoons of butter. Add the finely chopped onion and sauté for about 3-4 minutes until fragrant and translucent, filling your kitchen with a sweet aroma.
In a large mixing bowl, whisk together the cream of mushroom soup, shredded cheddar, milk, and Dijon mustard until smooth and creamy. Season with salt and pepper to taste.
Add the drained broccoli to the bowl and gently fold to coat all the florets evenly with the cheesy sauce.
Transfer the broccoli mixture into the prepared baking dish, spreading it out evenly with a spatula.
In a small bowl, combine the panko breadcrumbs with 2 tablespoons of melted butter, stirring until the crumbs are coated and slightly golden.
Sprinkle the toasted breadcrumb mixture evenly over the casserole, creating a crunchy topping.
Bake in the preheated oven for 20-25 minutes until the cheese is bubbling, the top is golden brown, and the breadcrumbs are crispy.
Remove from the oven and let the casserole rest for 5 minutes to set up before serving. The cheese will be gooey, and the top crispy—ready to enjoy!
