Heat the olive oil in a large pan over a medium heat and fry the onions and garlic for 5 to 6 minutes until slightly softened. Add the tomato purée and stir. Then add the beef mince, breaking it up with the back of spoon, and brown for a 4 to 5 minutes. Stir in the ‘nduja, fresh and tinned tomatoes, and the stock. Season with salt, freshly ground black pepper and the sugar. Add the torn basil leaves, then bring to a simmer and cook for 45 minutes to 1 hour until slightly reduced.
Meanwhile, make the white sauce - mix together the crème fraîche, mascarpone, parmesan and a good grinding of black pepper in a bowl. Set aside until ready to assemble at the next step.
Heat the oven to 200C/180C fan/ gas 6. Spread a third of the beef ragu over the base of a rectangular baking dish (ours was 25 x 18cm), then layer in three lasagne sheets. Spoon on a third of the white sauce, then add a handful of grated mozzarella or cheddar. Repeat to make three layers of each, finishing with the remainder of the white sauce. Top with the rest of the grated mozzarella, the torn ball of mozzarella and the extra grated parmesan.
Cover with tin foil and bake for 45 minutes, then uncover, reduce the oven temperature to 180C/160C fan/gas 4, and bake for 15 to 20 minutes until the cheese is bubbling and golden.