Cook the stomach, the deer lungs, heart, kidneys and liver in boiling water for two hours. Skim regularly. Remove the meat from the pot and cool.
In a large pan, sweat the onions with the butter over medium heat. Set aside. In a separate pan, cook the apples also in butter. Add the maple syrup and the grated nutmeg. Caramelise for 25 minutes. Set aside.
Use a meat grinder to grind the lungs, the heart, the kidneys, the liver and the chopped beef suet. In a large bowl, combine the ground meat with the onions, the caramelised apples and the roasted oat flakes. Fill the deer stomach with this stuffing. Tie the stomach closed with a string and pierce it in various spots with a knife.
Poach the stuffed stomach in hot but not boiling water (80°C) for about one hour.
Preheat oven to 375°F
Remove the stomach and place it in a large cast-iron saucepan. Cover the haggis with some of the sauce and pour the rest in the pan. Cook in 375°F oven for 40 minutes. Baste regularly with the sauce.
Combine an equal quantity of diced tomatoes, concentrated meat stock and beef broth. Stir with a hand mixer.