Preheat the oven to 400 °F. Lightly spray a foil-lined baking sheet with nonstick spray.
Stir together all the vinaigrette dressing ingredients in a small bowl and set it aside.
Cut up the parsnip, sweet potatoes, and red onion.
Slice the top off a head of garlic.
In a large bowl, toss the parsnip, sweet potatoes and red onion with 3 teaspoons (1 tablespoon) of olive oil, and a small pinch of salt and pepper. Spread the vegetables in a single layer on the prepared baking sheet.
Put the head of garlic on the baking sheet and pour 1 more teaspoon of olive oil over the exposed garlic cloves.
Roast in the oven for 30 minutes. At the 15-minute mark, give the parsnip, sweet potatoes and onion a good stir and add the cherry tomatoes to the baking sheet.
At the 30-minute mark, you'll probably want to remove the vegetables from the oven. The head of garlic might need 5 to 10 minutes more. Roast the garlic until the exposed ends of the garlic cloves are lightly browned. The garlic cloves should be squishy inside.
Squish out the garlic cloves and add them to the rest of the vegetables. Toss the vegetables with the maple mustard vinaigrette dressing and enjoy.