Place diced onion into a large microwaveable mixing bowl. Add butter and cook on high for 2 minutes until onion is translucent. Add mince, egg, herbs, ketchup, chilli and Worcester sauce.
With clean hands mix the mixture together and slowly add the breadcrumbs. Keep adding them til the mixture is firm but still sticky/moist. Cover mixture with cling film and place in fridge for a minimum of 30 minutes - longer the better.
Mix all the sauce ingredients together in a jug or bowl.
Transfer mixture to a lightly greased loaf tin and bake in oven at 200 C / Gas 6 for about 1 hour. Check after 30 minutes and drain any fat that has come to the surface.
Ten minutes prior to finishing (at 50 minutes) turn the meatloaf out onto a baking tray, and pour over the glaze and return to the oven for the final 10 minutes.
Allow to stand for 5 minutes before slicing and serving.