I’m in a constant battle with trying to maintain/gain weight and it’s not easy. I’d recommend baking some coconut/teff/buckwheat/millet bread and eating two/three slices a day, along with large portions of veggies for lunch or dinner. Oat bran for breakfast helps a little, as well as consuming olive oil. Maybe have some buckwheat groats with your veggies a couple times a week too. Just try to eat as much as you can during the day.