In a mixing bowl, stir together the tuna, eggs, onions, potatoes, ½ cup of the breadcrumbs, mustard, lemon juice, dill, salt, paprika and pepper. Mix well, cover and refrigerate 30 minutes.
Place the oil and butter in a large skillet over medium heat. Divide the tuna mixture into 8 portions and shape into balls. Roll in the remaining breadcrumbs, then mash into patties. Fry in batches for 3 minutes, then carefully turn and fry the other side until golden brown. Drain on paper towels.
Toast the buns and evenly smear each with mayonnaise. Add the croquette, top with lettuce and serve warm.
Recipe by The Tennessee Magazine at https://www.tnmagazine.org/bring-on-the-burgers/