Onion sets

Onions don't shade each other much so I think a close spacing without rows, like in square foot gardening, is fine. 10 cm is more than enough for decent sized bulbs, 15 cm and you'd get huge ones.

For plants which have much more foliage, I've found the SFG spacing doesn't work as well. Last year I grew beetroot and carrots in rows instead and they did much better.