In this article written by Juicing Journal, I was asked to contribute my favorite smoothie ingredients to assist with weight gain.
I recommend including avocados in smoothies for extra omega-3 fatty acids, calories, and creaminess. You can dice up a ripe avocado and freeze it so you can add a few cubes at a time to your smoothie without worrying about it going bad. This is a great trick especially when you see your avocados getting a little too ripe. This way you can freeze them and use them at another time for smoothies!
Check out the full article for more protein shake ingredients to help with weight gain: https://www.juicingjournal.com/best-protein-shakes-for-weight-gain/