Start to cook the cauliflower, broccoli, and pasta. I used my multi-cooker but you can cook these in separate pans if you don't have one. Whilst this is cooking, start on your sauce.
Heat some oil in a pan and add the flour. Stir until it resembles breadcrumbs
Add in the milk a little at a time, stirring/whisking well until smooth
Mix in the mustard and cheese (keeping a little cheese back for topping). Allow to simmer
Mix the pasta and vegetables into your sauce, making sure everything is well covered.Put into an over-proof tray/dish, sprinkle the remaining cheese on top, and cook in the oven or under the grill to melt the cheese.