Rumbledethumps is a traditional dish from the Scottish Borders. The main ingredients are potato, cabbage and onion. Similar to Irish colcannon, and English bubble and squeak, it is either served as an accompaniment to a main dish or as a main dish itself.
Cooked leftovers from a roast meal can be used. However, to make fresh rumbledethumps one needs to lightly sauté the shredded onion and cabbage in butter until the onion is transparent and the cabbage wilted, then add some potatoes mashed with butter, salt and pepper; after thoroughly mixing the ingredients, they are placed into an oven proof dish, and cheddar (or similar) cheese placed on top, if desired. This is then baked until golden brown on top.
An alternative from Aberdeenshire is called kailkenny which replaces the butter in the potatoes with cream.
We usually mix shredded cheese into it with the cabbage and onions; the "cheese placed on top if desired" is a must, too. A dash of nutmeg, and and a dose of granulated garlic mixed in with the S&P kicks it up a notch.
Finished product:
Chop an onion, and shred cabbage whiole boiling the spuds to mash, and the oven is heating to the 350-400F range. Saute them over low-med heat in enough butter to mash the potatoes, until the cabbage is limp & the onion is starting to turn translucent.
Drain & mash the potatoes, then add the onion & cabbage (and shredded cheese if using) & maybe sonme extra butter; don't forget the milk, half & half or ceream & the seasonings while mashing!
Put in an oven proof dish and spread it evenly and cover the top with cheese; garnish with (as in the pic) with red pepper rings, or other, as & if, desired. Bake until the cheeses is just starting to brown. (We use the same 8X13 deep loaf pan as for shepherd's pie.)
This is extremely forgiving, and can be varied almost endlessly.
Similar is Tatties & Neeps, in which mashed turnip is used in addition to, or instead of, onion--or even instead of the cabbage--your choice.
If my English to English translation is correct, it gets its name because you rumble (mix in) the cabbage & onion into the thumps (mashed potatoes, because you have to thump'm good to get'm mashed.