Final answer:
The vegetable related to onion and garlic with a mild, sweet flavor is the shallot.
Step-by-step explanation:
The vegetable related to onion and garlic that has a mild, sweet flavor is the shallot. It belongs to the Allium family, which includes onions, garlic, leeks, and chives. Shallots have a slightly milder taste than onions but still provide a unique sweet and savory flavor to dishes.