Preheat the oven to 390°F (200°C).
Wash and chop all the vegetables into bite-sized pieces, ensuring uniformity for even roasting.
In a large mixing bowl, toss the chopped vegetables, cherry tomatoes and garlic with 4 tablespoons of olive oil, salt, and pepper until evenly coated.
Spread the vegetables out in a single layer on a baking sheet lined with parchment paper or aluminum foil.
Roast the vegetables in the preheated oven for 30 minutes, or until they are tender.
Meanwhile, peel and slice the onion. Toss the onion slices with 1 tablespoon of olive oil.
After 30 minutes of roasting the vegetables, add the prepared onion slices to the baking sheet, distributing them evenly among the other vegetables.
Continue roasting for an additional 20 minutes, or until the onions are golden brown and the vegetables are tender and nicely browned.
Once roasted to perfection, remove from the oven and garnish with freshly chopped parsley before serving.