Preheat oven to 180C. Pour the passata into a large baking dish and set aside.
Heat a knob of butter in a large frying pan. Add the onions and fry for 2-3 minutes, before adding the spinach. Turn the heat down and cook for around 4 minutes, then stir in half of the cream cheese. Cook until the cream cheese has dissolved, and take the mixture off the heat. Using a hand blander or a food processor, blitz to form a smooth sauce. Taste and season with salt and pepper as required.
Spoon the spinach mixture into your cooled pasta shells, packing them quite full. Place them into the baking dish on top of the passata. Add about half a teaspoon of the remaining cream cheese.
Bake in the middle of the oven for 30 minutes. Serve warm.