Add the oil to a large saucepan or Dutch oven and heat over a medium setting.
Saute the onions, salt, red pepper flakes, and black pepper, for about 5 minutes or until the onion is translucent.
Add the chopped celery and minced garlic, continuing to stir as you saute.
Add the potatoes to the saucepan and cook, stirring constantly, for about 5 minutes.
Now, add the chopped green onions and corn, and cook for 2 minutes to heat.
Add the flour, and stir to coat all the vegetables.
Stir in 2 cups of the stock and bring the mixture to a boil, stirring until it thickens. Stir in the remaining 2 cups of stock until the mixture is smooth.
Reduce heat, cover and simmer for 15 minutes until the potatoes are almost done.
Add the cream, and continue to simmer, uncovered for about 5 more minutes.
Stir in the salmon and lemon zest gently, so the salmon remains intact. Cook for about 3 to 4 minutes until the salmon is just done.
Add the lemon juice and green onion, stirring gently just before you serve.