Just because it’s coming up to autumn there’s no need to stop planting in the garden. The other evening I stuck in a couple of rows of the onion sets I got last weekend. They’ll overwinter well and hopefully give us some tasty onions come early summer next year.
They’re a piece of cake to sow, normal preparation of the ground as per and preferably there’s been no compost been put in the ground for a season or two (if there was, they’d make the ground hold moisture over the winter thus leaving the sets open to rotting which isn’t good). All you do is make a hole with a dibber (or the other end of a trowel) and carefully put the set in with the top just very slightly poking out (and the root end downward).
I also stuck some sticks and brambles on the top to stop our cats digging them up and also hopefully keep the birds and squirrels away. I plant mine closer together then the usual (about 4″ apart) so I can thin them out and use them as small onions when they’re ready.
Never mind what they say about “onions being cheap in the shops so don’t bother growing them” as home grown onions are a hundred times better than the ones in the supermarket. Grow your own as they’re cheap as chips to buy as sets!