Whisk together 6 cups water, ⅓ cup salt, cracked peppercorns, and 1 teaspoon lemon zest. Add salmon fillets, cover, and let stand 15 minutes.
Meanwhile, remove lid from sorghum and cook, stirring, about 5 minutes until excess liquid has evaporated. Off heat, stir in remaining tablespoon oil, remaining teaspoon zest, peas, scallions, tarragon, and lemon juice; season with salt and pepper.
Transfer salmon fillets to a paper-towel-lined plate and pat dry. Combine 1 teaspoon each salt and pepper. Sprinkle half of salt and pepper mixture in bottom of a large nonstick skillet. Arrange fillets, skin side down, in skillet; heat to medium high and sprinkle with remaining salt and pepper over top.
Cook fillets until fat begins to render and skin starts to brown, 6–7 minutes. Flip, cook until flesh side starts to brown, 6–8 minutes more. (If you don’t have a 10-inch or larger skillet, cook fillets in batches.)
Serve fillets with leek purée and sorghum pilaf.