What is a shallot?
Related to the onion (as opposed to being a younger version of it), shallots grow in clusters at the leaf base. Most varieties are smaller than onions, have finer layers and contain less water.
The flavour of shallot is much milder and sweeter than that of an onion, so if a recipe specifies shallots, substituting onions won’t give the same results. Their lower water content means they need to be cooked more gently than onions.