Run your fingertips over each salmon fillet to check for any stray bones, pulling out any that remain between the flakes of fish. Arrange the leeks in a single layer in the bottom of a large shallow flameproof casserole dish with a tight-fitting lid. Lay the pieces of salmon on top. Surround the fish with the asparagus and sugar snap peas. Pour in the wine and stock, then dot the margarine over the fish. Season with salt and pepper.
Bring to the boil, then cover the casserole dish and reduce the heat so the liquid simmers gently. Cook the fish and vegetables for 12–14 minutes or until the salmon is pale pink all the way through and the vegetables are tender. Garnish with chives, then serve.