Add the olive oil, sumac, and salt and pepper, and toss well to combine using your hands or kitchen tongs.
Spread the vegetables out in a single layer on a large rimmed sheet pan lined with parchment paper for easier cleanup.
Roast the vegetables for 25 to 30 minutes until tender when pierced with a fork and lightly browned, stirring half way through. Serve immediately.
Tips for cutting the vegetables:
Broccoli, cauliflower, and Brussels sprouts cook at about the same rate when cut the same size.
Carrots take a little longer, so cut them in quarter-inch coins.
Onions take even longer to roast than carrots, so cut them in thin slices.
If you are cooking something else in the oven, you can roast the vegetables at the same time, as long as the oven temperature is between 400° and 450°F. The higher the temperature, the quicker they will take to cook, so keep your eye on them so they don't burn.
Spread the vegetables out in a single layer and do not overcrowd the pan to ensure they roast instead of steam.
Storing: Leftovers can be covered and refrigerated for up to 4 days. They can be reheated in the oven or microwave. Roasted vegetables are also delicious served cold in a salad.