Preheat the oven to 400 degrees F and line a rimmed baking sheet with aluminum foil.
Rinse the quinoa under cold water. Drain well and add to a saucepan. Add 2/3 cup water and 1/8 teaspoon salt and bring to a boil. Cover the saucepan, reduce the heat to low and cook the quinoa until tender and all water is evaporated, about 15 minutes. Remove from the heat and let sit, covered, for 5 minutes. Add the scallion and fluff with a fork.
Meanwhile, toss together the broccoli, tomatoes, garlic, oil and salt and pepper to taste on the prepared baking sheet until the vegetables are lightly coated with oil. Roast for 10 minutes.
Sprinkle the salmon on both sides with salt and pepper. Remove the baking tray from the oven and nestle the salmon into the vegetables. Roast until the vegetables are tender and the salmon is cooked through, 12 to 15 minutes more.
Divide the quinoa between two bowls. Top with the broccoli and tomatoes and lay the salmon on top.