Toast the oatmeal in a dry skillet over medium-low heat for 8 to 10 minutes, stirring often, then cool.
Simmer the lamb shoulder, liver, and heart in water to cover for 60 minutes. Drain, cool, and mince with the onions into a coarse irregular mixture.
Mix and Steam:
Combine the minced meat, onions, suet, toasted oatmeal, stock, salt, pepper, coriander, nutmeg, and allspice. The mixture should be moist but not soupy.
Pack loosely into a greased pudding basin, cover tightly with parchment and foil, and steam over gently simmering water for 2 hours. Add water to the pot as needed.
Rest 15 minutes, loosen the edge, and turn out. Slice or spoon onto plates.
Notes
This accessible basin method keeps the classic oatmeal-and-offal character without requiring a natural casing.
Serve with mashed neeps and tatties; refrigerate leftovers for up to three days.