Healthy high calorie foods

Don't laugh, but I'm actually wanting to know of healthy foods that are HIGH in calories, that aren't just completely loaded with bad sugars and fats.  I realize it's a somewhat contradictory request, but surely there are some foods out there that are calorie heavy, that are also good for you somewhat.  I know of the usual suspects like rice and potatoes, but those are things that aren't real easy to eat in the middle of the day at work.  Are pb&j's my best bet?

 I am one of "those" people that have a hard time putting weight on, and with heavy training I have to really watch out that I don't train myself into inexistance... quite literally.  I have a fairly healthy weight at the moment of 148-150 at 5'10", but if I'm not careful I will find that weight dropping pretty quickly.  When I get below 148, it seems my training starts to suffer and extreme fatigue starts setting in since my body is not liking the lack of fuel.  Then I go weight myself and find out I've lost some weight so I rush to the fridge and start gorging myself with whatever I can find even if I'm not hungry.  I'd really like to end this cycle and get more into a routine that will maintain necessary caloric intake.

 I know the simple answer is to eat, but if you're not hungry a lot of the time it's hard to do.  So I'd like to make the most of the times when I am hungry, eating foods that are heavy on the calories side so that I am getting the proper fuel I need to train long and hard!

Surprisingly, I think my appetite was a lot larger BEFORE I started training extensively, which is odd, but what are you gonna do.