Preheat the oven to 425 degrees and line a large baking sheet with parchment paper.
In a large bowl, whisk together the maple syrup, olive oil, Dijon mustard, minced garlic, rosemary, salt, and black pepper.
Add the carrot pieces, parsnip pieces, sweet potato cubes, and red onion wedges to the bowl. Toss until the vegetables are evenly coated.
Spread the coated vegetables in a single layer on the prepared baking sheet.
Nestle the sausages among the vegetables on the same sheet.
Roast for 25 to 30 minutes, turning the sausages and stirring the vegetables once halfway through, until the vegetables are tender and the sausages are browned and cooked through.