Place the tomato pasta sauce in a saucepan and heat through, then stir in the ham, basil and courgette and season.
Meanwhile, prepare the lasagne sheets, if necessary, according to the pack instructions. Mix together the crème fraîche and measurement water in a bowl until smooth.
Spread a third of the tomato sauce over the bottom of a medium-sized ovenproof dish. Drizzle a quarter of the crème fraîche over the sauce, then top with a third of the lasagne sheets, cutting to fit the dish, if necessary. Repeat with the remaining ingredients, finishing with the remaining crème fraîche, and scatter with the Parmesan.
Place in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes or until bubbling and cooked through.