Drain the canned salmon thoroughly, pressing out excess liquid. Transfer to a bowl and flake with a fork, removing any bones or large pieces of skin.
Finely dice the yellow onion and green bell pepper into small pieces (1/4 inch or smaller). Slice the green onions thinly. Add all vegetables to the bowl with the salmon.
Add flour, cornmeal, beaten egg, Worcestershire sauce, paprika, garlic powder, onion powder, mustard powder, cayenne, salt, and pepper. Mix until evenly combined. If too wet, add an extra tablespoon of flour or cornmeal.
Divide the mixture into 8 equal portions. Press each firmly into a flat patty about 3/4 inch thick and 3 to 3.5 inches across. Chill in the refrigerator for 10-15 minutes to firm up.
Pour oil into a cast iron skillet to a depth of 1/4 to 1/3 inch. Heat over medium-high heat until it reaches 350°F (175°C), or until a pinch of cornmeal sizzles immediately.
Fry in batches of 3-4 patties, cooking undisturbed for 3-4 minutes per side until golden brown and crispy. Use a fish spatula for easy flipping.
Transfer cooked croquettes to a paper towel-lined plate to drain. Allow oil to return to temperature between batches.
Serve hot with your choice of dipping sauce (tartar, remoulade, or comeback sauce) and your favorite Southern sides.