When it comes to building muscle, the most important thing is training. Even in situations that aren't ideal, the body can build muscle to survive the threat imposed by training. Research has shown you absolutely can build muscle in a calorie deficit, as long as protein intake is high (1). And anecdotal evidence from prisons shows that guys get jacked all the time, despite awful nutrition.
This is not in any way suggesting that eating isn't important. No skinny guy will gain a respectable amount of muscle without proper nutrition. However, you need to have your priorities straight. Muscle building starts with hard training, and perhaps surprisingly, hard training will improve your eating.
One of the biggest obstacles a true hardgainer has is a pathetic appetite. Hard training is one of the fastest, easiest ways to increase your appetite. It makes your body demand more food and this makes eating large amounts of food way easier.
Oddly enough, your appetite can actually help you find your training sweet spot:
The right amount of hypertrophy training will leave you ravenous!