Preheat oven to 425°F. Toss sweet potatoes, fennel and red onion on a sheet pan and drizzle with 2 tablespoons olive oil. Season with salt and pepper and toss. Spread into a single layer and roast 30 minutes or until sweet potatoes are almost tender.
While the vegetables are roasting, pat salmon dry and brush both sides with 2 more tablespoons olive oil. Season with salt and pepper. When vegetables have roasted 30 minutes, turn and push to the side, clearing a space in the center of the pan for the salmon. Return the pan to the oven and roast until salmon is just beginning to turn opaque in the center, about 5-8 minutes.
While the salmon is roasting, combine the vinegar, mustard and 1/3 cup olive oil in a jar with a tightly fitting lid. Season with salt and pepper and shake until well combined. Put the greens in a large bowl and toss with 2-3 tablespoons of the dressing.