In a medium bowl, combine the drained tuna, breadcrumbs, egg, mayonnaise, lemon juice, chopped parsley, and diced onion. Mix well until all ingredients are thoroughly combined.
Season the mixture with salt and pepper to taste.
Shape the mixture into patties.
Heat olive oil in a frying pan over medium heat.
Cook the patties in the heated oil until golden brown, about 3-4 minutes per side.