Heat the butter in a saute pan over a medium heat and gently saute the onion and garlic until they begin to soften, about five minutes. Set aside to cool slightly.
Place lamb mince, onion, garlic, oatmeal, spices and egg in a bowl. Using your hands, mix well until thoroughly combined.
Transfer the lamb mixture to a piece of non-stick baking paper, and press out into a rectangle shape slightly wider than your loaf tin, and to a depth of about 3/4 of an inch.
Crumble your haggis into smallish pieces and sprinkle over the surface of the lamb, pressing it in slightly.
Using the edge of the baking paper to help you, roll up into a swiss roll shape. Squeeze the lamb mixture at the ends to seal in the haggis.
Transfer the meatloaf to a large loaf tin and cover in foil. Place the loaf tin into a larger, deep casserole dish and pour in boiling water to a depth of one inch.
Place in the middle of the oven and bake for 45 minutes.
If you want a browner meatloaf, remove the meatloaf from the tin at the end of the cooking time, turn the oven up to 200 C and cook for another five minutes or so.
Serve with a nice whisky sauce, and some tatties & neeps.