Why is onion considered as a stem?

Hint: Here to get the exact answer of this question at first we have to go through the process of learning from the beginning that is first we need to understand the basics of onions and its growing procedures. Common onions are usually available in the three colours varieties: Yellow or brown colours, and red or purple colour.

Complete answer:
The onion bulb is commonly known as the bulb onion or common onion that is the most widely cultivated species of the genus Allium is a kind of a vegetable. A botanical variety of the onion is the shallot. A fan of bluish-green leaves, hollow is a characteristic of an onion plant. As an annual it is grown and it is like a biennial plant. The onion bulb is a modified stem because it consists of large numbers of fleshy scales leaves. At the base of the onion the root appears as a thread like structure. Having a cluster of fleshy leaves, the onions look like a tunicate bulb. These are cultivated and used all round the world.

Note:
The bulbs of the onions seem to be shortened, compressed, underground stems. Usually the onion grows to a height of 15 to 45cm. The world production of the onions and the shallots was 4.5 million tonnes in 2019, that was led by China with $22\%$ of the world total and Japan,Mali, and South Korea as secondary producers.