Preheat the oven to 220 C.
Trim the brussels sprouts and cut them in half.
Cut the cauliflower into florets.
Add them to a large bowl as you prep them.
In a small bowl, combine the onion powder, garlic powder, salt and pepper.
Toss the vegetables to cover them with spices.
Spread the vegetables on a baking tray and place them in the centre rack of the oven.
Bake for 15 minutes, then toss the vegetables.
Continue roasting them for another 5-10 minutes until they are crispy outside and tender in the middle.