Preheat the oven to 400°F (200°C). Toss the diced sweet potatoes and trimmed broccolini with 1 tbsp of olive oil, salt, and pepper. Spread evenly on a baking sheet and roast for 20-25 minutes until tender.
While the vegetables roast, combine the rinsed quinoa and 2 cups of water in a medium saucepan. Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes until the water is absorbed.
In a small bowl, whisk together the grainy mustard and maple syrup. Brush this mixture evenly over the top of the salmon fillets.
Heat 1 tbsp of olive oil in a large skillet over medium-high heat. Add the fillets skin-side down and sear for 4-5 minutes per side until cooked through.
In another small bowl or jar, combine the remaining 1 tbsp of olive oil, lemon juice, salt, and pepper. Whisk vigorously to create the lemon vinaigrette.
Assemble by dividing the cooked quinoa evenly among four containers. Top each with roasted vegetables and a seared fillet. Drizzle the lemon vinaigrette over the top before sealing.