Soak the sausage casing in salt water overnight.
Rinse the liver, heart, and tongue, and cook them over medium heat in a large pot of boiling, salted water for 2 hours.
Remove any gristle, skin, or blood vessels, then mince these organs into 1/2 inch pieces.
In a large bowl, combine the minced parts with the suet, onions, oats, and dry seasonings.
Remove the sausage casing from the water and fill with the mixture.
Tie the casing closed with butcher’s twine, trim off any extra length from both sides, then pierce it several times with a fork.
Bring a large pot of water to a boil, then gently lower the haggis into the pot.
Reduce heat and simmer for 3 hours.
