Preheat the oven to 200C/400F/Gas 6.
Heat the oil in a frying pan over a medium heat. Add the onion and garlic and fry until softened.
Add the leeks and cavolo nero and cook for 3-4 minutes.
Add the squash, tomatoes, wine and season, to taste, with salt and freshly ground black pepper. Simmer for 5-6 minutes, then divide between four individual ovenproof dishes or ramekins.
For the topping, place the flour, baking powder, salt and butter into a food processor and pulse until the mixture resembles breadcrumbs.
Add the soured cream and pulse again until it just comes together as a very sticky dough.
Spoon dollops of the topping over the contents of each ramekin, then bake for 25 minutes, or until crisp and golden-brown.
For the dressing, place the oil, basil and lemon juice into a clean food processor and blend until smooth.
Serve the cobblers at the table in their ramekins and spoon over the dressing.
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