Preheat your oven to 375°F (190°C).
In a large bowl, mix the rye flour, water, and salt together to form a dough. Knead the dough for 5-10 minutes until it becomes elastic.
In a separate bowl, mix together the diced pork belly or bacon, diced whitefish, chopped onion, salt, and pepper.
Divide the dough into 2 equal portions. Roll out each portion into a large circle (about 14 inches or 35 cm in diameter).
Place one circle of dough into a greased baking dish. Add the pork, fish, and onion mixture onto the dough.
Cover the mixture with the other circle of dough, crimping the edges of the dough together to seal the filling inside.
Cut a small hole in the top of the dough to allow steam to escape during baking.
Bake for 2-3 hours or until the bread is cooked through and golden brown.