Add broccoli and onion to the baking dish with a good drizzle of oil and a hefty pinch of salt and pepper. Use your hands to combine, then roast in the oven at 200C/390F for 15-20mins, or until the broccoli is lightly charred and fork tender. Give it a mix halfway.
Meanwhile, place pasta in salted boiling water and cook until al dente. Pasta should be a touch hard still, if it’s fully cooked it’ll come out sloppy when baked.
Add butter and garlic to a large pan over medium heat. Stir in flour to form a roux, then gradually add in milk, whisking as you go to avoid lumps.
Whisk in veg stock and simmer for a few mins, or until the sauce begins to thicken.
You should be able to run your finger across the back of the wooden spoon and have it leave a path clear.
Stir in mustard, nutmeg and a good pinch of salt and pepper, then add in the broccoli/onion. Add in cheese then stir in the drained pasta.
Pour everything back into the baking dish and level off with a wooden spoon.
Top with breadcrumbs and more salt and pepper, then liberally spray with oil. Bake in the oven at 200C/390F for 20mins, or until the breadcrumbs and golden all across the top.