Chop onion in food processor. Add fish. Whirl until coarsely ground. Scrape into bowl. Add red pepper, celery, salt and black pepper. Shape into 4 equal burgers. Coat with bread crumbs.
Heat oil in large nonstick skillet over medium-high heat. Add burgers; cover and cook until golden brown, about 1-1/2 to 2 minutes per side. Remove from skillet. Serve on buns with ketchup, if desired.