Peel and roughly chop the onions, dice the lamb into 3cm chunks, then divide both between four 15cm ovenproof bowls, placing the bowls on a large oven tray.
Add 1 teaspoon each of mint sauce and umami paste to each of the bowls, followed by 150ml of water and a little pinch of sea salt and black pepper. Stir well.
Quickly scrub the potatoes and rattle them through the thick slicer attachment of a food processor so they’re just under ½cm thick.
Divide between the bowls, overlapping them slightly to cover. Press down on the potato layer to compact everything, then cover with tin foil and bake for 2 hours, removing the foil for the last 30 minutes.