In a medium mixing bowl, whisk together egg, horseradish, mayonnaise, and dijon mustard.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add minced garlic, green onions, and sweet peppers. Sauce until vegetables are tender, about 2-4 minutes.
Add sautéed vegetables to the bowl with the flaked fish. Season with garlic salt and add the egg mixture. Lastly, add in the plain breadcrumbs and use gloved hands to mix it all together.
Place panko breadcrumbs on a shallow plate.
Form fish mixture into 3 patties. Press each patty into the panko breadcrumbs to coat both sides.
Add enough vegetable oil to a large skillet to come about 1/2-inch up the sides. Heat over medium-high until the temperature reaches 350 F.
Carefully add fish patties to the hot oil, being careful not to crowd them. Cook for 3-4 minutes per side, just enough to get them nicely browned. Remember, the fish is already cooked! Remove to paper towels to drain.