Ground Beef heart if you have the filtration for it. I have a mix I make, and though my jags are tiny, I give several packs to a friend who has a large jag and it loves the stuff.
Mine eats frozen MUSSCLES I just get them out of the fridge and drop them in a cup of boiling water when they are soft and warm dangle them in the tank he nearly takes my fingers off for them. He also eats frozen prawns but would rather have a MUSSCLES he goes mad for them follows me up and down the tank as soon as I turn up with the cup,
I feed mine nightcrawlers and it loves them. I also feed them frozen krill, beefheart, chickenliver, and frozen vegetable cubes (from the LFS). The nightcrawlers are neat to watch getting eaten and dont worry if it looks too long your jag will make quick work of it.
My Jag has never turned down anything we've offered him.
Beefheart, nightcrawlers, shrimp, and smelt will beef it up though I inject the food with a pea/garlic mix too.
My jag will eat anything. I feed krill, pellets, flakes, freeze dried bloodworms tubifex worms frozen shrimp smelt beefheart blood worms, live blackworms trout worms ghost shrimp and of course the occasional rosy.