Preheat the oven to 200°C/fan 180°C/gas mark 6. Heat the oil in a frying pan and cook the onion for 5 minutes until it begins to soften.
Stir in the turkey and cook for a further 5 minutes until it begins to brown. Stir through the kidney beans, tomatoes and herbs, and simmer for 5-10 minutes.
Meanwhile, make the white sauce according to the instructions on the packet, then stir together with the crème fraîche.
Spoon a third of the turkey mixture into an ovenproof dish. Cover with a single layer of pasta sheets and spread a spoonful of the white sauce over the top. Repeat twice, finishing the final layer with the remaining sauce. Top with the mozzarella and bake for 30 minutes, until golden.