Preheat your oven to 400°F (200°C).
On a large sheet pan, toss the sweet potato, Brussels sprouts, and red onion with 1 ½ tablespoons oil, minced garlic, salt, and pepper.
Roast in the oven for 20-25 minutes, or until the vegetables are golden and tender.
While vegetables roast, rinse the lentils and place them in a pot with the vegetable stock.
Bring to a boil, then reduce heat and simmer for 15-20 minutes until tender but not mushy.
Drain any excess liquid from the lentils.
In a small bowl, whisk together the ingredients for the vinaigrette: olive oil, cider vinegar, maple syrup, and Dijon mustard.
In a large bowl, combine the cooked lentils and roasted vegetables.
Drizzle the vinaigrette over the salad and toss gently to coat.
Serve warm or at room temperature.