Bring a large pot of water to a boil and fit it with a steamer attachment. Steam the potatoes for about 10 minutes, or until fork-tender (you can also boil them till fork-tender). Remove the potatoes from heat and allow them to cool for 10 minutes. Place them in a large mixing bowl, cover, and transfer to the fridge. Allow them to cool completely.
Cut the avocados in half, remove the pits, and then scoop the flesh into a medium-sized mixing bowl. Mash with a fork. Add the mustard, lemon juice, salt, and black pepper to taste. Mash thoroughly, until the avocado mixture is relatively smooth.
Remove the potatoes from the fridge and add the dill and onions to the bowl. Fold the avocado mixture into the potatoes until everything is fully incorporated. Season to taste and serve.
Gena Hamshaw is a certified nutritionist, recipe developer, and food blogger. She shares her latest culinary adventures at The Full Helping. She's the author of two cookbooks, Food52 Vegan (2015) and Choosing Raw (2014). She enjoys yoga, sweet potatoes, cashews, and things that are smothered in sauce.