01 - Preheat oven to 375°F (190°C). Steam broccoli and cauliflower florets until just tender, then drain thoroughly.
02 - In a medium saucepan over medium heat, melt butter. Add flour and whisk continuously for 1 minute to form a smooth roux.
03 - Gradually add milk to the roux, whisking constantly until the mixture thickens and becomes smooth.
04 - Remove saucepan from heat. Stir in 1 cup shredded cheddar cheese until melted, then season with salt and black pepper.
05 - Gently fold the drained vegetables into the cheese sauce to coat evenly. Transfer mixture to a greased baking dish.
06 - Sprinkle the remaining 1/2 cup of cheddar cheese evenly over the top.
07 - Bake for 20 minutes or until the casserole is bubbling and the cheese topping is golden brown.