Pre-heat oven to 400 degrees and line a baking sheet with foil. Spray foil with cooking spray and set aside.
Place chopped veggies (sweet potatoes, cauliflower and onion) in a large resealable plastic bag and drizzle with balsamic vinegar, olive oil, salt and pepper. Seal tightly and shake well until vegetables are evenly coated.
Spread vegetables in an even layer on a baking sheet and roast until lightly browned and caramelized, 20-25 minutes.
Sprinkle with fresh herbs before serving.
The key to a deep, golden brown vegetable is not over-crowding the baking sheet. If you find the vegetables too crowded, use two pans. If the vegetables are too crowded you will end up steaming them, not roasting them.