Place salmon in a pot, and add leek, peppercorns, 2 teaspoons salt, and 4 cups water. Bring to just under a boil, reduce heat, and cook at a bare simmer until salmon is just cooked through, 3 to 5 minutes. Remove from water, and cover with parchment to keep moist.
Puree sliced basil and lemon zest in a food processor. Add sugar and lemon juice, and pulse to combine to make vinaigrette. With machine running, add oil and season with salt.
Bring a medium pot of water to a boil. Add 1 tablespoon salt and the peas. Reduce heat, and vigorously simmer until tender and bright green, about 4 minutes. Drain, and transfer to a bowl. Add 2 tablespoons basil vinaigrette, and crush with a potato masher. Season with salt and pepper, then add mint.
Divide peas among 4 plates, and top with salmon. Drizzle with more vinaigrette, and garnish with basil sprigs.