Beef liver is tough for two reasons. One, it is from an old animal and two, it has been over cooked and at too high a temperature. Slice the liver thick and dredge it in a seasoned flour of your choosing and fry it low and slow in hot lard until just barely cooked through. I use pork lard, because I have lots and I want to caramelize my onions in the seasoned liver bits when I am done with the liver.
To get rid of the bitter taste soak the liver in
milk. Lots and lots of milk. I have 4 jersey cows so I can afford to do it for 2 or three days changing the milk daily 2x if needed. If the liver is from an old or grain fed steer, the milk will turn into a gelatine like pink pudding. Rinse it off into the
compost bucket and pour on fresh milk. Keep soaking until the milk stops jelling. Fry as above. So tender almost too tender and no more bitterness because the milk will have drawn out the icky stuff. Seriously, butchers these days slice the liver too thin like minute steaks and its almost impossible to do a good job cooking it. Next time,try slicing your liver yourself about 1/2" thick.