01 - Preheat oven to 350°F. Lightly grease a 9x13-inch baking dish with butter or nonstick spray.
02 - Bring a large pot of salted water to a boil. Add broccoli florets and blanch for 2 to 3 minutes, until vivid green and just tender. Drain immediately and set aside.
03 - Melt 2 tablespoons unsalted butter in a large skillet over medium heat. Add chopped onion and sauté until translucent, about 3 minutes. Stir in minced garlic and cook for 1 minute longer.
04 - In a large mixing bowl, combine sautéed onion, garlic, blanched broccoli, cheddar cheese, sour cream, whole milk, condensed cream of mushroom soup, Dijon mustard, salt, and black pepper. Mix thoroughly until ingredients are evenly incorporated.
05 - Transfer the mixture into the prepared baking dish. Spread evenly to ensure a consistent bake.
06 - In a separate bowl, blend crushed cornflakes with 2 tablespoons melted unsalted butter. Sprinkle the mixture uniformly across the surface.
07 - Bake in the preheated oven for 30 to 35 minutes, until the casserole is bubbly and the topping is golden and crisp.
08 - Allow the casserole to rest for 5 minutes before portioning and serving.