Rinse fresh salmon; pat dry. To poach fresh salmon, in a large skillet, bring water to boiling. Add salmon. Return to boiling; reduce heat. Simmer, covered, for 6 to 8 minutes or until the salmon flakes easily with a fork. Remove salmon from skillet, discarding poaching liquid. Flake salmon into 1/2-inch pieces; set aside.
Meanwhile, in a 4-quart Dutch oven, combine vegetable broth, onions, potato, snipped fresh dill, lemon zest, salt and pepper. Bring to boiling; reduce heat. Simmer, covered, for 15 minutes or until vegetables are tender, stirring occasionally.
In a large screw-top jar, combine milk and cornstarch. Cover and shake well; stir into soup. Stir in asparagus. Cook and stir until slightly thickened and bubbly. Cook and stir for 2 minutes more. Gently stir in poached salmon or canned salmon; heat through. If you like, garnish with fresh dill sprigs and/or cracked pepper.