dice small a few onions - sweat them off in a saucepan (on gentle heat with lid on - steam dropping in will be enough to stop any burning).
Add Lean minced beef to sweated onion - keep lid off but keep it turned and bash out lumps of beef whilst gently browning off meat. Add a few beef oxo cubes crumbled up (then no need for extra seasoning) to the mix - keep it moving on a low heat.
Drain off any excess fat (should not be much with lean minced beef).
Cook off potatoes "Maris piper" best if you can get them - cream them when cooked with salt, pepper, butter (not fake butter) and a little milk until the mash is soft but not to slushy stage.
Now you have two choices -
either, as I prefer, just spoon the beef mix into a casserole pyrex type dish and spread or pipe potato mash on top - make pretty patterns in the top of mash - top with grated strong cheddar - bake until golden on top (approx gas 3 slow and steady)
OR, as above but add some beef gravy into minced beef mixture - if you prefer a moist version.
Hope this helps mate, good luck - not much can go wrong ???