Normally, when you think about building muscle, you think protein. But that shouldn't be the only thing on your plate.
To gain mass, you have to ensure you're getting enough protein to rebuild muscle tissue damaged through training, but you also have to eat a lot of carbohydrates because gaining size requires you to fill your muscles with glycogen.Glycogen is the collection of carbohydrates stored in muscles that powers your training and affects anabolism; think of it as an energy stockpile. To maintain that supply as you're trying to gain muscle, you need a slight positive intake in carbohydrates at each meal -- that is, you have to eat more carbohydrates than you burn.
Unsaturated fats, especially olive oil, are essential. They help all kinds of functions in the body that affect growth. Plus, they're a dense source of calories, which is an important factor in adding weight.
Vegetables are another must. "People think they're a diet food, but they're important because of their fiber content." Fiber helps cleanse the intestines, and some speculate this can increase nutrient absorption, yet another consideration in growth and tissue repair.