Cook the rice according to the package directions and keep warm.
Position a rack 4 inches from the broiler and heat the broiler on high.
In a small bowl, stir the miso, mirin, and rice vinegar with a fork to blend. Put the salmon fillets skin side down on a foil-lined rimmed baking sheet, leaving space between them. Pat the salmon dry and broil for 2 minutes. Remove the baking sheet from the oven and, with a spoon, carefully spread the miso mixture over the top of the fillets. Broil until the salmon is just barely opaque in the center (use a paring knife to check), 2 to 3 minutes more.
Meanwhile, in a small saucepan, bring 2-1/4 cups of water to a simmer. Put the tea leaves and 3/4 tsp. salt in a 4-cup heatproof liquid measuring cup. Pour the hot water over the leaves and let steep for 1 minute.
Gently mix the scallions and sesame seeds into the rice and divide among four large shallow bowls, mounding it in the center. Pour the tea through a strainer around each mound. With a spatula, lift the salmon from the baking sheet, leaving the skin behind, and place on top of the rice. Sprinkle with more sesame seeds and serve immediately.