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

I think that should be ok.

To be honest Spec I think the notion that they shouldn't be planted together is twaddle.

The argument being that it stops the beans producing nitrogen to give back to the soil. Recent studies have shown that the nitrogen realised is only equal to what it uses to break the roots down in the first place.

Most of the ammonium nitrate (NH4) is used by the plant when setting seed