Preheat your oven to 400 degrees F (200 degrees C). 2. Grease a 10x14x2-inch baking pan thoroughly. 3. Heat 2 tablespoons of butter in a skillet over medium heat. 4. Add the chopped onions to the skillet. 5. Cook and stir the onions for about 5 minutes, until they are soft and translucent. 6. Reduce the heat to low. 7. Continue to cook and stir the onions for an additional 40 minutes, until they are very tender and dark brown. 8. Bring 4 quarts of chicken stock to a boil in a large pot. 9. Stir in the 1.5 kg of chicken breasts. 10. Reduce the heat and simmer for about 20 minutes, until the chicken is no longer pink in the center. 11. Remove the pot from the heat. 12. Use a slotted spoon to transfer the cooked chicken breasts to a plate. 13. Reserve the chicken stock in the pot. 14. Juice the 4 kg of potatoes using an electric juicer. 15. Place the dry potato pulp into a bowl and discard the potato juice. 16. Stir 2 tablespoons of salt and 1 tablespoon of black pepper into the potato pulp. 17. Add enough reserved chicken stock to the potato mixture to achieve the consistency of oatmeal. 18. Set aside any remaining chicken stock. 19. Spread half of the potato mixture evenly into the prepared baking pan. 20. Lay the cooked chicken breasts evenly over the potato layer. 21. Scatter the caramelized onions evenly over the chicken. 22. Spread the remaining potato mixture over the onions and chicken to cover completely. 23. Bake in the preheated oven for about 1 hour, or until the top is golden brown. 24. Reheat any remaining chicken stock. 25. Pour the hot chicken stock over individual servings as desired.