Hi Tara,
Great question! Let’s start with shallots. Substitute 3/4 cup of minced shallots for the 1 cup of diced onions.
We wouldn’t recommend scallions, unless you use them as a garnish.
If leeks are not an issue, they would be a great substitution in equal amounts (leeks and potatoes are a classic combination). Be sure to use only the white and light green parts of the leek. Cut off the root end and the tough dark green parts (can save for making fresh stock). Slice down the center lengthwise and rinse to remove dirt and sand. Then thinly slice into half moons.
Let us know how it turns out.
Thanks for the question,
Joel and Mary