Preheat the oven: Preheat the oven to 425°F and line a large rimmed sheet pan with parchment paper for easy cleanup.
Cut the vegetables: Peel and cube the sweet potatoes into bite-sized pieces, halve the Brussels sprouts, and cut the red onion into wedges, keeping all pieces similar in size.
Slice the sausage: Slice the chicken sausage into coins about half an inch thick so it roasts evenly alongside the vegetables.
Toss the vegetables: Place the sweet potatoes, Brussels sprouts, and onion on the sheet pan, drizzle with olive oil, and season with salt and pepper, tossing to coat evenly.
Spread in a single layer: Arrange the vegetables in a single layer with space between pieces so they roast instead of steam.
Start roasting: Roast the vegetables alone for 10 minutes while you prepare the sausage and glaze.
Add the sausage: Remove the pan, add the sliced sausage among the vegetables, and return to the oven for 15 to 18 minutes, until the edges turn golden brown.
Whisk the glaze: In a small bowl, whisk together the Dijon mustard, maple syrup, and minced garlic until smooth.
Glaze the pan: Pull the sheet pan out of the oven and brush or drizzle the maple Dijon glaze over the sausage and vegetables.
Finish roasting: Return the pan to the oven for 3 to 5 minutes, until the glaze turns glossy and slightly sticky.
Rest before serving: Let the pan rest for 2 to 3 minutes off the heat so the glaze sets slightly before serving.