Drain each can of tuna completely; flake with a fork into a bowl.To the tuna add the diced red onion, eggs and seasonings. Mix thoroughly with a spatula until fully combined and holding together; refrigerate at least 30 minutes. Heat 1/2 of the butter in a saucepan over medium high heat until melted. Turn half the mixture into 4 tuna cakes and cook 3-4 minutes per side. Flip each cake and repeat until golden brown and heated through. Remove from the pan and repeat with 2nd batch of tuna cakes. Use a dollop of  mayo with each tuna cake.