Creating a barrier

I want to plant some onions but apart from planting where I have leeks at present, the only other bed I have is where I will be growing peas and beans which apparently are not compatible together, if I split my bed into three section  by growing brasicas in the middle section would that act as a barrier between the onions and the legumes, the bed is 10feet long by 9feet wide and not walked on,  so no need for paths