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Bring a large pan of salted water to the boil, tip in the carrots and boil for 3 mins, then add the sprouts and boil for another minute before adding the leeks. Boil everything for 5 mins more, then drain well.
Sizzle the butter in a large frying pan or sauté pan until it turns a nutty brown, then add the sage, if using. Let it crackle, then transfer to a plate, leaving the butter in the pan. Add some cracked black pepper to the butter, tip in the vegetables and gently stir-fry until the veg is piping hot and starting to brown around the edges. Season with sea salt and serve straightaway, with the sage scattered over, if using.