Preheat the oven to 380F / 190c.
Place a large pot of water on the stovetop and set it to boil. Add in a few pinches of salt.
Cut the broccoli and cauliflower into florets. Once the water is boiling, add them in and cook for 3 to 5 minutes til tender, but still firm.
Drain the broccoli and cauliflower and set to one side.
Place the butter in a small pot and place in on a medium low heat on the stovetop to melt it.
Once melted, add in the flour and stir well til the flour is incorporated and you have formed a paste. Keep stirring and cooking for 5 minutes to cook the taste of the flour out.
Take the pot off of the heat, and add in the milk in increments, stirring all the time to incorporate.
Place the pot back on the heat, and increase the heat to medium. Keep stirring the whole time until the sauce starts to simmer. Cook for another couple of minutes until you have a thick sauce - stir all the time!
Take the sauce off of the heat and add in the mustard, a few pinches of salt and black pepper, and 2 cups (200g) of the shredded cheese. Stir to combine.
Place the broccoli and cauliflower in your baking dish. Pour over the cheese sauce, making sure that the tops of the florets are fully covered. Sprinkle over the remaining cup (100g) of shredded cheese.
Place in the oven for 25 to 30 minutes to bake. The cheese sauce should be bubbling and the top should be browned.