Bring 1 inch (2.5cm) of water to a boil in a pot with a steamer basket. Add the thick sliced orange carrots and steam for 2 minutes. Add the white cauliflower chunks and bright green broccoli florets and steam for an additional 3-4 minutes until lightly tender.

Remove vegetables from the heat, drain any excess water, and spread them evenly into a rustic white ceramic baking dish to form the base foundation.

In a heavy-bottomed saucepan, melt the unsalted butter over medium heat. Whisk in the all-purpose flour and cook for 2 minutes until it becomes pale golden and smells faintly nutty.

Gradually whisk in the warm whole milk. Continue cooking and stirring for 4-5 minutes until the sauce thickens enough to coat the back of a spoon. Whisk in the sea salt and mustard powder.

Remove the saucepan entirely from the heat. Gradually stir in 2 cups of the freshly grated cheddar cheese until it melts into a thick, rich, creamy yellow-orange sauce.

Preheat oven to 375°F (190°C). Pour the cheese sauce evenly over the steamed vegetables, tossing gently so they are thoroughly coated. Sprinkle the remaining 1 cup of cheddar cheese evenly over the top layer.

Bake uncovered for 20-25 minutes until you see a beautifully golden-brown, lightly charred baked cheese crust with bubbling, crispy edges clinging to the side of the dish.

Let the casserole rest for 5-10 minutes on a smooth neutral kitchen counter. Garnish by perfectly scattering tiny flecks of freshly cracked black pepper and finely chopped green parsley leaves across the melted glossy cheese before serving.