Seems like everyone has that one food they never run out of when they're trying to grow. Mine has been ground beef lately because it's easy to cook in bulk. Curious what everyone else keeps stocked that makes hitting calories easier.
Eggs. Cheap, easy and I can eat them basically any time of day.
Rice, hands down. I cook a giant batch on Sunday and just build meals around it all week. Makes getting those extra carbs in way less of a headache.
Greek yogurt is always in my fridge. Throw in berries, granola, honey, whatever I’ve got around and it’s an easy snack when I’m short on calories.
Peanut butter.
One spoonful turns a “damn, I’m still 500 calories short” day into a much easier problem.
Chicken thighs have been my go-to lately. More flavor than breast, pretty cheap, and they reheat without turning into rubber. I’ll cook a big tray and portion it out with rice and veggies.
Ground beef has been clutch for me too. I usually cook a big batch with some seasoning, then switch up the carbs/sauces so I’m not eating the exact same meal five days straight. 
Oats are the one for me. Breakfast, shake, late-night bowl doesn’t matter. Super easy way to bump carbs and calories without feeling like I’m forcing down another huge meal.
Honestly, bagels. They’re stupid simple when calories are getting hard to hit. Toast one up, add eggs or some nut butter, and boom, another solid meal without spending half the day cooking.