Prep oven: Preheat the oven to 400 degrees (200 C)
Prep vegetables: Cut stems from the onion and then make a slice around the outside the onion to remove the top layer. Then cut the onion into rounds, or in half and then into slices. Cut the stems from the cauliflower and remove the leaves. Wash the head of cauliflower, then slice in half and then use a knife to cut the florets from the thick white center.
Roast vegetables: Arrange the cut cauliflower florets and onion slices on a roasting pan and drizzle with olive oil, tossing the vegetables to completely cover with oil and sprinkle with sliced garlic, salt, and pepper. Place sheet pan in the oven and roast for 25 minutes, stirring partway through to brown on all sides.
Serve vegetables: Remove pan from oven, arrange vegetables on a plate and serve as a side dish or let cool and use as a salad or bowl topping.
