i think the best way to put healthy weight on is to eat what a dietician will advice you
better see one.
because a temporary solution is not a solution
u can go to a pharmacy and buy yeast extract kind of thing and will put on weight very quickly but once u stop it, u lose the weight again the same quickly.
the dietician will give you the most efficient list of food both for putting on that weight and also to have it gained solidly, also with best benefits regarding the other health issues.
with any kind of wrong diet with someone's advice you can still put on weight, but may give damage to your kidneys, liver, hearth etc. or you may just be putting the weight on as fats..

sooo... best is to see a dietician..

good luck.