From what I've read on a bunch of fitness forums, whether you're trying to gain weight or lose weight, everyone who is serious about their health basically eats the same 12 things.
Chicken breasts
Lean cuts of beef
Eggs
Fish: Salmon / cod / halibut
Brown rice
quinoa
Sweet potatoes
Whole wheat bread / pasta
Nuts
Healthy oils (olive oil / avocado oil)
Any green leafy vegetable (broccoli, spinach, green leaf lettuce, brussels sprouts, kale)
Avoid sugar, drink lots of water. Add a protein shake with milk if you're having trouble getting enough calories. It'll probably take you 4 months to gain 15 lean lbs.