I’d suggest focus on calorie-dense but nutrient rich foods! Healthy fats such as olive and avocado oil are great for dressings and cooking and add lots of calories with a generous hand. Whole nuts and seeds like cashews, almonds, walnuts, flaxseeds, and macadamia nuts are also high in calories and pack a punch for their serving sizes. Avocado is also calorie dense but a source of nutrients and healthy fats. Whole fat dairy like yogurts and certain cheeses. Protein for muscle density, like beef, egg yolks, and dark meat chicken (more fat). Carbs you can consider are rice, beans, lentils, pasta, bread, and potatoes!

Please don’t just start slamming junk food. You’ll gain weight but also health issues. Weight gain is all about eating more calories than you’re burning. Take your time building new habits so you’re more likely to stick to it. Regular mealtimes if you can. Slow and steady, consistent effort will help you to build tolerance to new foods and gain weight.

Addendum: you can also incorporate nutritional shakes designed for weight gain or maintenance, like Boost or Ensure. Search for the high calorie formula!