Bring a saucepan of water to the boil and blanch the onions for 5 minutes to soften. Drain and transfer to a bowl. Add the rapeseed oil, Dijon mustard, sugar and season with salt and freshly ground black pepper. Set aside.
Cook the potatoes in a pan of boiling, salted water for about 20 minutes or until they are just tender. (Be careful not overcook them as you want a bit of resistance in the potatoes.) Drain the potatoes in a colander, return them to the pan and dry them out over a very low heat.
Gently heat the milk. While the potatoes are warm, crush them roughly in the pot with the back of a fork. Add the hot milk and 60g/2¼ oz butter.
Boil the kale in hot water for 4 minutes, then drain thoroughly. Add it to the crushed potatoes and season with salt and pepper. Set aside.
Fry the bacon in a dry frying pan until crisp. Drain on kitchen paper and chop into small pieces. Fry the reserved kale leaves in a dry pan until crisp and charred.
For the liver, heat a little vegetable oil in a frying pan until very hot. Place the liver in the pan with the bruised sage. Season with salt and pepper and add the remaining butter and 2 cloves of garlic. Cook for about 3-4 minutes on both sides, regularly basting with the melted butter. Remove the liver from the pan and set it aside to rest.
Add the kidneys to the pan with the cumin and remaining garlic. Flash-fry for 2 minutes turning frequently.
Divide the kale and potatoes between serving plates and top with the onions. Slice the liver and place on top with the kidneys and drizzle with juices from the pan. Garnish with the bacon pieces and crisp kale leaves.