Remove bones and any skin from salmon. Flake apart. Mix together with the onion, egg yolks, lemon juice, and cracker crumbs. Beath egg whites until stiff and then fold into the salmon mixture. Put into a casserols dish that has been lightly greased. Bake for 30 minutes in a 300º oven.
Mix the melted butter with flour. Add the milk and heat over medium heat until sauce thickens. Season to taste with salt and pepper. Serve over souffle .