1. Melt butter in a soup pot over medium heat and cook onion, celery, and bell pepper, stirring. Add garlic after vegetables start to get soft, 3 to 5 minutes, and cook 1 minute. Add flour and stir until it is evenly dispersed. Add potato, carrots, broth, dill, salt, and black pepper. Stir and bring to a boil over high heat. Reduce heat to medium-low and simmer until potatoes are fork tender, about 20 minutes.
2. Add milk, corn, and salmon. Heat 5 more minutes. Ladle into bowls and serve with a little grated cheese on top, if using.