Place the tuna, mashed potatoes, corn and egg in a large mixing bowl. Sprinkle onion powder, paprika, salt and pepper on top.
Mix everything really well with your hands. Alternatively, if you have a food processor, use it to pulse the mixture until well combined.
Scoop some of the mixture out with your hands and shape into cakes. They should be the size of small cookies, 2.5 to 3 inches (6–7.5 cm) across. You could use an ice cream scoop or ⅓ cup measure to portion the mixture before shaping into patties if you want them to be of uniform size.
Coat both sides of each fishcake in breadcrumbs.
Chill the fishcakes in the fridge for at least 30 minutes before cooking (this helps to firm them up). Or pop them into the freezer for 10 minutes.
Melt a few tablespoons of coconut oil in the frying pan on medium heat until nice and hot. Then place a few fishcakes in the pan and let them cook on one side until golden brown. Don't crowd the pan - the fishcakes are easier to flip and manouver on the pan if they are not crowded together.
Once they get golden and crispy on the first side, flip them gently onto the other side (it takes about 3-4 minutes to cook each side). Add more coconut oil between batches as necessary. The fishcakes should cook in about 1/4 inch oil so they don't stick to the pan.
Serve hot with a squeeze of lemon juice on top and some buttered sourdough bread.