Hmm, counting calories is really going to be your best bet but eating lots of large meals throughout the day will do it! 20-30 lbs is quite a lot - that would be 70,000 surplus calories!!! If you can, start lifting heavy. Make sure you eat at least 1g/lb of protein a day. Protein and fats and carbs, eat lots of them. Don't do any cardio.
High-quality sources of fats and protein are going to be your friends- avocados, nuts, beef, barley, and full-fat dairy are all great options, but there are hundreds more. Also fun stuff like pasta, fortified cereals, and fortified breads. And never ever neglect ur fiber intake- that will also help ur body adjust to your revamped dietary plan.
"Healthy" is just a buzzword with no one correct definition. If you're 20lbs underweight, then a 1000 calorie breakfast made of twinkies and bacon IS the healthy choice. Healthy is what's good for YOUR body. Your body doesn't care if the calories come from a package or a farm, it just needs them. Lots of them. Energy is energy.