Melt the butter with the oil in a heavy-based saucepan over a low heat. Add the onion and celery and cook, stirring occasionally, for 10 minutes until softened but not coloured. Stir in the passata and sugar and bring to the boil. Reduce the heat and gently simmer, uncovered, for 10 minutes until thickened. Season with salt and pepper.
Cook the ravioli in a large saucepan of salted boiling water according to the packet instructions until al dente. Meanwhile, add the cream to the sauce and bring to the boil. Stir in the nutmeg and remove from the heat.
Drain the pasta thoroughly and transfer to a serving dish. Spoon over the sauce and serve immediately with a scattering of Parmesan shavings and garnished with basil leaves.