Stir together oil, rosemary and garlic in a small bowl. Place potatoes in a large bowl and toss with 1 tablespoon of the oil mixture and 1/2 teaspoon each salt and pepper. Arrange the potatoes in an even layer on a large rimmed baking sheet. Roast until lightly browned and tender, about 20 minutes. Push the potatoes to one end of the pan.
Place asparagus in the large bowl and toss with 1 tablespoon of the oil mixture, 1/4 teaspoon salt and 1/8 teaspoon pepper. Arrange the asparagus on the other end of the baking sheet. Roast until the asparagus is bright green, about 3 minutes. Push the vegetables to either end of the pan, leaving space in the center.
Brush salmon with the remaining 1 tablespoon oil mixture and sprinkle with the remaining 1/4 teaspoon salt and 1/8 teaspoon pepper. Place the salmon in the center of the pan. Thinly slice half the lemon and tuck the slices around the salmon and vegetables. Roast for 5 minutes more. Cut the remaining lemon half into wedges.
Whisk balsamic glaze and mustard in a small bowl. Brush 1 tablespoon of the mixture on the salmon. Continue roasting until the salmon is just cooked through and the vegetables are tender, about 5 minutes. Drizzle the vegetables with the remaining sauce. Serve with the lemon wedges.