I haven't, but that's a neat idea. Are you trying to make square onions or are you just being creative with what you have laying around?

(I knew a woman who grew plant, I don't remember which ones but largely flowers I think, in holes in cinder block. She had this place along an alley and there was like six inches between the fence and the asphalt where weeds were prone to grow, so she set up a row of cinder blocks and filled them with potting soil and planted things.

Onion roots tend to grow outward for a bit and then turn downward. I don't recall how far out they grow. Someone here had a link to drawings a few months ago I think.

Yeah, the root system of most plants are surprisingly extensive and large. I will link below to a very good old study made many years ago about root systems of veggie crops. When you go there, scroll down to find onions and click on that.

I have a long row of those blocks with two holes in each block. They grow unwanted grass and weeds well but I have tried to grow a few flowers and veggies and found that it is not worth the effort in our area because of the difficulty in trying to keep them watered. If the bottom of the block was sitting on the ground so that the roots could go on down into the ground, it might work satisfactorily, or if you have more time, patience and lower expectations than me, it might work for you. See link below.
Bill P.

Here is a link that might be useful: Root development in vegetable crops

I may have 2 things going in my favor here. First, my blocks are stacked 2 high so that will give me a little depth. The second thing is that the blocks are pretty much in-ground-meaning there is very little of the block above ground...the beds are pretty deep. I think I'm going to give it a shot for the Spring planting. After all, gardening is all about experimenting. I'll keep everyone posted hopefully with pictures.

Think I'd choose something other than bulb onions. Also, they take about 4 months to mature. As a practical point, onione are quite cheap in stores and flavors are just fine. Maybe a few non-bulbing green onions would be nice. That way you could just harvest the few you might use at any one time.