Preheat oven to 400°F.
Wash and dry potatoes. Use a fork to piece each side of the potatoes 5 times. Place in microwave safe bowl filled with approximately ½-¾ cup water. Microwave on high for 5 minutes. Flip potatoes over. Microwave an additional 5 minutes. Remove from bowl, let cool 10 minutes. When cool enough to handle, peel with your hands.
Mash potatoes in a medium-sized bowl. If they’re lumpy, add up to ¼ cup milk to help smooth them out. It’s okay if some lumps still remain.
Mash tuna in a separate bowl with a squeeze of lemon juice. Then, add tuna to the mashed potatoes along with the paprika, salt, pepper, parsley, oregano, and grated onion. Mix well to combine. Taste, and adjust seasoning as needed.
Add the eggs and mix well to combine.
Pour bread crumbs into a wide, shallow bowl or Tupperware container. Use a large spoon to scoop tennis-ball sized amounts of the tuna mixture into your hands, and shape them into balls. Roll each ball in the bread crumbs to coat. Place the balls in a greased casserole dish, then gently flatten into discs approximately ½″ high.
Bake tuna cakes at 400°F for 45-50 minutes, flipping halfway through.