Peel and chop the potatoes into roughly equal sized chunks. Simmer the potatoes in a pan of boiling salted water for 15 mins.
While the potatoes cook, remove and discard the tough stalks from the kale. Finely slice the leaves. Peel and finely chop the onion. Slice the mushrooms.
Add the kale to the potatoes. Simmer for another 4-5 mins till the potatoes are tender and the kale is just wilted. Drain the veg. Tip them back into the pan. Season well and mash till the potato is smooth. Cover with a lid to keep warm.
Warm ½ tbsp olive oil in a deep frying pan or wok. Add the onion and mushrooms. Season with salt and pepper. Fry for 5 mins till the veg are soft and slightly coloured. Stir often.
Add the mashed potatoes and kale. Turn the heat down to medium low. Cook for 15 mins till caramelised and browned. Scrape and stir every so often so you get crunchy bits. Cover when it’s cooked to keep warm.
While the bubble and squeak cooks, rub 1 tsp oil into each lamb leg steak. Season. Warm a griddle or frying pan over a medium heat. Fry the lamb for 4 mins on each side (cook for a few more mins for medium to well done). Pop on a warm plate. Cover with foil to rest.
Turn the heat down under the lamb pan. Pour in 1 tbsp of the balsamic vinegar. Stir and cook for 1-2 mins to deglaze the pan.
Slice the lamb steaks. Serve with the bubble and squeak and the balsamic pan juices poured over the top.