Green apple flavors are usually from acetylaldehyde which can form a couple of different ways. You could have underpitched your yeast, not given enough time in primary or not fully oxygenated your wort. If you have already kegged it there is nothing left to do but hope that it fades with time.
Kegging at 1.022 seems pretty high...even though the gravity reading was stable, the yeast could have stalled out and just quit working...if you want to try to fix it, you could bleed off the excess CO2, rack back into another vessel and re-pitch some of the same yeast species. Or you could just chalk it up to lesson learned and improve on the next batch.
-Sometimes I feel like I'm up beer creek without a pint glass-
-May the IBU's be with you-