Heat oven to 450 degrees F. On a rimmed baking sheet, toss together the broccoli, potatoes, and onion with the oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Spread in an even layer and roast for 15 minutes.

Season the salmon with 1/4 teaspoons each salt and pepper, nestle it among the vegetables and continue roasting until the vegetables are golden brown and tender and the salmon is opaque throughout, 8 to 10 minutes more.

Meanwhile, in a small bowl, combine the mayonnaise, lemon juice, and garlic. Serve with the fish and the vegetables.