James Kilkelly writes in his Irish garden blog that:

“Onions are members of the Allium family and a popular crop for those of us who grow our own food. There are three main groups of onions: salad or spring onions, which are sown in the autumn or spring, the autumn sown varieties for early bulbs, and the early sown kinds for late keeping.

Their flavours range from mild to strong and extremely strong or real tearjerker. All parts of the plant are edible but we generally restrict ourselves to the bulb.

Top tips from garden know how tell us that:

“It is just one of those onion facts that the key to growing big onions is early planting, with fertilizer or compost. Seeds can also be sown in trays and left in a cool location until the seedlings reach about 1-2 inches tall, at which time they can be placed in deep biodegradable pots filled with loose, composted soil. Place seedlings at the top and keep pots somewhat dry to encourage more extensive rooting as they move down in search of moisture.

Plant the pots in the garden in early spring, and as they absorb moisture from the soil, they will eventually decompose, encouraging a secondary root system near the soil surface, which will produce larger onions.”

Read more at Gardening Know How: Onion Info – Tips For Growing Big Onions https://www.gardeningknowhow.com/edible/vegetables/onion/onion-info-tips-for-growing-big-onions.htm