Mix the rice and 2 1/3 cups water in a medium skillet. Bring to a boil. Reduce heat to low and simmer, covered, for 10 minutes.
Slice the salmon on a diagonal into four 3/4-inch-thick pieces. Season with 1/4 teaspoon salt and 1/4 teaspoon pepper. Place on top of the partially cooked rice. Cover and cook 7 minutes.
Scatter the peas over the salmon and rice. Cover and cook until the rice and peas are tender and the salmon is opaque and beginning to flake, 3 to 5 minutes.
Mix the soy sauce, scallions, vinegar, ginger, and sugar in a small bowl. Spoon the sauce over the salmon, rice, and peas and serve.
Technique: You can adapt this one-pot cooking method to shrimp and scallops. (Add them with the peas, after the rice has cooked for about 15 minutes.) The key is covering the skillet, so everything steams together. If your pan doesn’t have a lid, use a baking sheet.