Heat oven to 425°F. In bowl, whisk mirin, soy sauce, vinegar, brown sugar and half each of garlic and ginger.
On rimmed baking sheet (make sure it’s level and unwarped for even cooking), stir together rice, 2 ½ cups water, remaining garlic and ginger and ¼ teaspoon salt. Bake 10 minutes.
Meanwhile, in medium bowl, toss broccoli with olive oil and 1 tablespoon ginger-soy mixture. Stir rice, scatter broccoli on top and bake 6 minutes.
Nestle salmon amid rice. Spoon 1 tablespoon of remaining ginger-soy mixture over each piece and bake until salmon is opaque throughout, 8 to 11 minutes.
Meanwhile, into remaining ginger-soy mixture, stir sesame oil and sesame seeds, then half of scallions. Serve salmon and broccoli rice drizzled with scallion-soy vinaigrette and sprinkled with remaining scallions.