Well, you can, Murrie, but I wouldn't. Onions like lots of sun and the brassicas are big leafy plants so will shade out the onions. I would plant the onions on the south side of the brassicas.

Vegetables are usually planted in rotation round 3 or 4 areas of your garden. To see why this is visit the link below which will help you understand the different needs of your different crops. http://www.gardeningdata.co.uk/vegetables/crop_rotation/crop_rotation.php