I work in an office every day and commute an hour and a half half to work, so cooking time and ease of preparing the meals is really important to me. Over time I've come up with these simple meals that I put together that all cook while I'm at work. Remember that the slow cooker is your friend.
The cheapest and simplest thing I make as far as main course is a roast chicken all in the slow cooker. All I do take a chicken that I bought at the store, spice it, and put it in the slow cooker while I'm at work. I set a timer on the slow cooker so that it only cooks 6 hours. When I come home it's still warm and it's pretty much ready to eat. Interesting thing about slow cooking a chicken is that the chicken meat becomes really fragile and you can make a really good pulled chicken out of it. This means after I had my drumsticks that first night and I shred the rest of the meat, which is really easy, so I can put it into the containers I'll be using for the next 2 or 3 days for lunch. The morning of work I'll take one of the containers out and pour a sauce on it whether that's a homemade or storebought BBQ sauce, honey mustard, red ranch, or even a homemade salsa. I then put some greens on the side to convince myself I'm eating enough of them. I eat this generally at room temperature, so after I take it out of the fridge it stays out on my desk until I'm ready to eat it for lunch.
If you've got a little bit more time you can use the pulled chicken to make chicken salads and those are absolutely fantastic cold. I personally adore curried chicken salads (i copykat the whole foods one) and could eat that for lunch almost every day of the week.
The same technique can be applied to a pork shoulder which I can use to make pulled pork after slow cooking. However I don't personally like cold pork as much as chicken.
The best part about all of this to me is that I don't have to worry about overcooking it; I don't have to worry about cooking it at all because it's happening while I'm at work. And taking what's coming out of the slow cooker and making something every day, that's actually pretty easy.
I'm going to rattle off a few other main dishes I make for lunch at work though I won't explain them: Steak salds, cold tomato soup, half a baguette and cheese, just about any appetizer you'll find at a nice party, sandwiches with cold cuts (keep mayo or other sauces separate until you are ready to eat so you don't have a soggy mess), and pasta with pesto (it's not too bad reheated in a microwave).