Best way to get away from being underweight is to eat foods that you've been lied to about and ones that have been conveniently forgotten.
• ruminant liver 1-2 times per week
• low fat seafood helps maintain trace minerals such as selenium
• plenty of sugar from sources such as orange juice, milk, dates, and honey
• traditional ice cream that is made only of milk, egg yolks, sugar, and a flavoring such as cocoa powder or coffee
• collagen hydrosylate (processed gelatin) + taurine + creatine in milk as a protein shake
• strict avoidance of excessive polyunsaturated fat will help the weight go to the right places—muscle instead of fat.
• after gaining enough weight, moderating total fat intake is the way to avoid becoming overweight.
• eating ruminant flesh in general is an effective way to increase protein consumption.
• starches such as white rice and potatoes are fine Only the potatoes are nutritious by themselves. especially if you struggle to get extra calories
• stick to greens that you digest well (probably something like spinach) and cooking will aid digestion. Brassica can be more problematic.
• random vegetables in general are not necessary—especially when you eat liver—and they may only serve to fill your intestines with low calorie junk and fiber that feeds intestinal bacteria—not you—and lower appetite for foods that will help you put on muscle
• a carrot or two a day is fine though
• get plenty of salt. Iodized salt can be problematic though—especially when you get enough iodine from seafood and milk.