Heat olive oil in a large saucepan, over medium heat.
Add chopped onion and saute until onion is soft and translucent, about 10-12 minutes. Stir frequently so onions don't brown.
Add sauerkraut and mushrooms. Season generously with pepper. Stir to combine.
Cover and simmer over low heat for 45 minutes until mixture is soft, stirring frequently. Add a teaspoon of water if mixture gets too dry and starts to stick to the bottom of the pan.
Using an emersion blender, puree the mixture until fairly smooth.
Add breadcrumbs, paprika, and stir in additional pepper. Be generous with the pepper, filling should be tangy and sharp.
Season with salt if needed. Filling may not need any salt, since sauerkraut is fairly salty.
Set aside filling and allow it to cool completely.