You can't boil-off fat, only water. You can pour out the water that appears after a few minutes of frying, but you'll be pouring out a lot of flavour as well as some (not all) fat.
If you buy a hand mincer (like
this or
this), you can then carefully select, steaks, beef joints or any other piece of meat that has less fat on it, cut it into strips (removing even more fat if you want) and mince it yourself.
A 21-day matured beef joint in Aldi costs about £8/Kg, so 500g of mince could cost £4, with no extra fat at all. That's probably comparable to extra lean minces from supermarkets, and they're probably not made from 21-day matured meat.
But something made with mince containing no fat will lack flavour and be dry.