For Ally as requested
Not really a recipe, you could probably have guessed
Do your normal amount of potatoes for mash. Meanwhile chop onion quite small.
Quantity will depend on number of portions, how strong the onion, and how oniony you want your mash, but for the two of us I did about 3/4 of an average sized onion. If it was a big Spanish onion I'd probably do a small half, they're milder aren't they, but you do want quite a lot, it should end up good and oniony, if it's meant to be onion mash
Cook the onion. Tonight I cooked the sausages in the oven so while it was on put the chopped onion in a dish with a knob of butter and covered it with foil and baked it. Otherwise I microwave it, the same, in a dish with a bit of butter.
Then you mash your potatoes, and add the onion with any loose butter - you probably still need a bit more butter, and maybe a little milk, maybe some salt & pepper.
I only really do them with sausages, and actually also like it with raw onion if it's to go with eg baked beans, but Bb's not so keen so I usually cook them