1. Soak: Submerge the sheep stomach completely in a bowl of salted water to soak overnight in the refrigerator before starting.

2. Simmer: Place the lung, liver, and heart into a saucepan with water to cover, draping the lung's windpipe over the side of the pan. Place a small basin underneath the hanging windpipe to safely catch any dripping impurities as it boils steadily for 1 1/2 hours, then drain and let cool.

3. Trim: Carefully remove and discard the windpipe along with any tough gristle or skin from the cooled lung tissue.

4. Grind: Feed the trimmed lung, liver, and heart through a meat mincer or grinder along with the lean mutton pieces.

5. Toast: Place the fine oatmeal into a dry skillet over gentle heat, stirring constantly until it turns pale golden brown and crisp.

6. Combine: In a large bowl, mix the toasted oatmeal together with the minced meat mixture, shredded lamb suet, and chopped onions.

7. Season: Add the salt and black pepper to the bowl, then pour in enough beef stock to moisten the entire stuffing mixture thoroughly.

8. Pack: Stuff the seasoned mixture into the prepared stomach bag, filling it just slightly over half-full so the swelling oatmeal has room to expand. Securely sew the opening shut or tie each end tightly with sturdy kitchen string.

9. Elevate: Place an upturned heatproof plate flat into the bottom of a large stockpot filled with boiling water to protect the casing.

10. Boil: Rest the stuffed bag directly on the plate and bring the water back to a boil. Carefully prick the casing all over with a large needle to prevent it from bursting, then boil steadily for 3 to 4 hours.

11. Respect the process and take it step by step; you've absolutely got this.