Cut the meat using a sharp filleting knife or other long-bladed knife so that you get a large, long piece, with a uniform thickness. Smear it on both sides with salt and black pepper. Put it on top of aluminum foil - a sheet larger than the meat itself.
For the filling, cut the leeks into thin round slices, the mushrooms into slices. Braise them in the butter until reddened. Season them with salt and black pepper to taste and distribute on top of the meat.
Wrap the meat carefully into a roll. Tie it well with thick, wet thread and wrap it in the foil.
Put it in a tray and bake about 1 hour 30 min. in a hot oven.
In the meantime, cut the vegetables - the peeled potatoes into sticks, the carrots into round slices, the leeks into round slices as well, but thicker and without separating the slices, the red onions into crescents. Mix them all together in a large oven dish, season with salt, black pepper, paprika. Pour in a little water and cut and add in the remaining butter. Bake them as well. Before they're fully ready, place the pork roll in the middle of them and finish baking until it's reddened.
After taking it out of the oven, sprinkle with dill. Before serving, remove the thread, cut it into slices and serve with a side of the veggies.