Heat the olive oil in a large frying pan. Add the chopped onion and stir until softened. Add the minced beef and stir until brown, drain any excess fat. Add the courgette, baked beans and chopped tomatoes, stir. Add the Worcester sauce, black pepper and mixed herbs. Crumble in the stock cubes and stir well. Cook on a low heat for 15 to 20 minutes, stir occasionally.
Meanwhile prepare the mash: Place chopped potatoes and sweet potatoes into a pan, cover with boiling water and bring back to the boil, simmer for 20 minutes.
Preheat oven to 180 C / Gas 4.
Drain the water, then add the milk and butter or spread and mash well.
Spoon the minced beef mixture into the bottom of a casserole dish and spoon the mash on top, smoothing it out. Mark all over with a fork.
Cook the shepherd's pie in the oven for 30 to 40 minutes, until the top starts to brown.