In a small bowl, combine the ground flax with 3 tablespoons. Set aside for 10 minute. It will thicken and gel up a bit.
Heat oil in a large skillet over medium heat. Add onion and sauté until softened and lightly browned. About 3-5 minutes.
Add garlic and sauté for 1 minute. Add sage and rosemary and sauté for 30-60 seconds until fragrant.
Add mushrooms and sauté until softened and slightly browned. Cook for 5-7 minutes until they reduce in size and release their moisture.
Add the soy sauce or tamari and cook for 1 minute. Remove from heat.
Add the chickpeas to a large food processor or blender. Pulse a few times to break down the beans.
Now add the cooked mushrooms, rolled oats, breadcrumbs, flax egg and tomato paste to the food processor or blender. Pulse until well combined and it almost forms a paste, but leave some texture. Taste and add more seasoning if needed. Place the mixture uncovered in the freezer for 20-30 minutes, or in the fridge for 1-2 hours.
Remove from the freezer and form into balls using your hands. Roll them until smooth.
Heat oil in a skillet and brown the meatballs. Remove from the skillet.
Spoon the sauerkraut into the same skillet and top with meatballs. Cover and let it simmer for 15-20 minutes or until the meatballs are cooked through, adding water if necessary.