Heat the grill to high and cook the meatballs for 12-15 mins on a baking tray or following pack instructions. Cut each one in half and set aside.
Tip the spinach into a large colander over the sink. Pour boiling water over to wilt it and leave to drain thoroughly. When cool enough to handle, squeeze out any excess liquid and chop finely. Mix the spinach with the pesto, ricotta, egg and nutmeg, then season to taste.
Heat oven to 190C/170C fan/gas 5. Pour the passata over the bottom of an ovenproof dish and stir in the garlic. Divide the spinach mixture between the pancakes, spreading it out in a long strip in the centre. Add meatball pieces to each one, then roll the pancake up to seal in the filling. Lay the stuffed pancakes on the passata base and top with the mozzarella. Bake for 30 mins until the cheese is melted and bubbling. Scatter over basil leaves to serve.