109 at 5'7" is very thin. I am that height at 154 lbs so I know (most people think I'm thin compared to others)
downgarden gave good ideas: eat more nuts and nut butters. Look in the healthfood store, there are more nuts than peanut butter
Smear nut butters on stalks of celery and top with raisins (what moms call "ants on a log"), put nut butter on sliced apples, etc.
Put cream in your coffee, put olive oil in everything you can think of, I LOVE coconut oil for cooking and it can be added to smoothies.
Smoothie: probiotic yogurt, fruit of your choice, honey, coconut oil, ice.
Homemade icecream using cream. Peanut butter pie. Put some nut butter on your daily chocolate treat.
Also hope you're eating a lot of protein to keep from losing muscle. Easy dinners are fattier cuts of meat put in the oven on low temps (250) all day. Meat cooks slowly and gets very tender with no work from the cook. Make a veg or salad and it's dinner.
Salmon (wild is best) has great omega oils in it too. More expensive but delicious.
I would keep gym to a minimum while trying to add weight, v. little cardio, concentrate on small weights for muscle toning. Building muscle is good but you don't want to burn calories with cardio. That can come later.
Good luck.