Most people plant onions as sets; semi-mature bulbs which will hopefully grow bigger and be harvested before they go to seed. But you can also raise onions from seed sown in your greenhouse instead of from sets planted later. This is especially useful on cold, wet soils or in cold, wet springs, as onions cannot tolerate those conditions. Seed-raised plants are cheaper than sets, you have more varieties to choose from, and the plants are less likely to bolt or be pulled out of the ground by birds. On the downside, seedling onions need a longer growing season so February is definitely the last month for sowing, and the longer growing season means more time for pests and diseases to attack them.
Sow the black, angular seeds thinly in a seed-tray, or in modules at up to five seeds each. Keep them at about 10-15°C. The seedlings come up in loops, and you should prick them out from seed-trays into pots before they pull their tips out of the compost and straighten up. If multiple seedlings come up in modules, either leave them to push each other apart later as they grow, or single them with a fine-pointed scissors to give the one remaining plant room to grow a bigger onion. Keep the compost neither bone-dry nor saturated. Grow on the plants in bright, cool conditions until they have two or three leaves, harden them off well and plant them out at about 15cm apart each way. Wider spacing gives you bigger bulbs, and tighter spacing gives smaller ones.
You can also plant sets in pots and give them a head-start under cover, planting them out only when conditions improve.