Heat the olive oil in a large saucepan set over medium heat. Add the onion, celery, and red bell pepper, and cook until the vegetables are beginning to soften, about 5 minutes.
Add the garlic, thyme and red pepper flakes, and cook for 1 minute.
Add potatoes and vegetable broth. Bring to a boil, then reduce heat slightly and simmer until the potatoes are tender, about 15 minutes.
Add the corn and simmer for 2 minutes.
Place the flour in a medium bowl and slowly whisk in the milk until the mixture is smooth.
Slowly whisk the milk mixture into the soup, along with the salt and pepper. Cook, whisking frequently, until the soup is starting to thicken, 8 to 10 minutes. Stir in the salmon and cook until the salmon is just cooked through, 6 to 8 minutes.