Leek (Best Option)
1/2 cup chopped leek per 1/2 cup chopped onionLeeks belong to the same allium family as onions and share many sulfur-containing compounds that create savory, aromatic flavors when cooked. Their texture softens nicely in long-cooked dishes like stew, contributing a subtle sweetness and complexity.
When substituting, use the white and light green parts, finely chopped. Clean thoroughly to remove grit. Cook them slowly to develop their sweetness and avoid any fibrous texture.
Compared to onions, leeks provide a milder, less sharp flavor, which can make the stew taste slightly sweeter and more delicate, but they maintain the essential aromatic base that onions provide.