In a large pot, heat oil over medium heat. Sauté onion, garlic, dill and 1/4 tsp (1 mL) pepper for about 5 minutes or until onions are softened. Stir in potatoes, corn and water; bring to a boil over high heat. Cover, reduce heat to medium-low and simmer for 5 to 10 minutes or until potatoes are almost tender.
Increase heat to medium. Whisk flour into milk and gradually stir into pot. Stir in salmon. Simmer, uncovered and stirring often but gently, for about 5 minutes or until salmon is firm and opaque and soup is thickened (do not let boil). Stir in lemon zest and juice and season to taste with pepper. Ladle into warmed bowls.