Combine all ingredients except onion, crab or tuna, bread crumbs, and oil in a bowl and whisk. Add crab or tuna, half the bread crumbs, and onion and mix thoroughly. Heat the oil in a frying pan over medium heat. Form the mixture into patties and bread with the remaining crumbs. Add to the oil and fry each side for five minutes. Do not flip more than once. Remove to a rack or paper towels.