Dice onion and put into a large microwaveable mixing bowl. Add butter and cook on high for 2 mins until onion is translucent. Add mince, and all other meatloaf ingredients.
With clean hands mix the mixture together and slowly add the dry bread crumbs. Keep adding them til the mixture is firm but still sticky/moist. Cover mixture with glad-wrap and place in fridge for a minimum of 30 mins - longer the better.
Mix all the sauce ingredients together in a jug or bowl.
Transfer mixture to a lightly greased bread tin and bake in oven at 200 degrees C for approx 1 hour - check after 30 minutes and drain any fats/oils that have come to the surface.
10 minutes prior to finishing (at 50 minutes) turn the meatloaf out onto a baking tray, and pour over the glaze/sauce and return to the oven for the final 10 minutes.
Allow to stand for 5 minutes before slicing and serving.