The name for this dish means "mixed together" - rumbled, and "bashed together" - thumped. This is a meatless main course but can also be served as a vegetable dish along with meat.
Slice the potatoes thickly and boil in a little salted water. Once cooked, drain and mash.
Slice the cabbage and boil gently in salted water, do not over cook! Melt the butter in a heavy bottomed pan and cook the onions or leeks. Once soft right through, add the cooked potatoes and cabbage. Add a little cream, season to taste and beat together.
Place the mixture in an oven safe dish, cover with grated cheaddar cheese and place under a hot grill or oven to brown.