One of two things...finish it anyway or save it for a little while later and finish it when you get your second wind of eating. Your metabolism is super fast so you are going to have to learn to eat, eat and eat some more. It might be hard at first, but you will get the hang of it. Just don't eat junk food or empty calorie items and you should be able to keep your BF% low while gaining mass although I doubt you will ever have to worry about your BF.
Stats: 340y/o, 7'5", 2250lbs, 4% bf, meso-endomorph. Working out and taking supps since the dawn of time.
Disclaimer: I am not real. I am a player on a virtual world rotating through cyberspace. Any information I give is based off of science fiction and not meant to be taken literally. I do not know anyone that uses, condones the use, or knows where to get anything against the laws of our governments.