When it comes to leek substitutes, there are a few options that can work well in its place. garlic, onion, shallot, and chives are all good choices. Each of these options will provide a similar flavor to leeks, albeit with some subtle differences.
Depending on the recipe you're making, one of these substitutes may work better than the others.
For instance, if you're looking for a leek substitute to use in a soup or stew, garlic or onion may be your best bet. If you're looking for a leek substitute to use in a salad or as a garnish, shallots or chives may be a better option.