Heat about a tablespoon of olive oil and butter in the pan
Fry 2 garlic cloves and an onion (sliced) just for a few seconds, until coated.
Add the mince and cook until it's browned.
Strain everything through a strainer to get rid of excess fat Return to pan.
Add 2 oxo cubes
1 tin of chopped tomatoes (strained)
Salt & pepper
A good teaspoon of mustard.
Today I also added home-made strained celery soup (about half a cupful). If it tastes ok, that's it. If you feel it still needs a "bite", add some ground chilli, not too much. Don't add too much liquid, you can judge yourself. It must be fairly moist but not sloppy. All you need for topping is the mashed potatoes and today I crumbled some potato crisps on top.
Temp and time. I find that at a temp of around 160 degrees C for a good hour or maybe a bit longer, or until the top is nicely browned. Important thing is that the mince is really hot. Just dig a teaspoon into the bottom of the dish and taste it. If it burns your mouth, it's cooked thoroughly.