In the meantime, melt a knob of butter in a large frying pan then fry the chard for 5 minutes or until wilted and tender. Season with the nutmeg, salt and pepper.
Press against the pan with a wooden spoon to squeeze out excess juices and pour them away. Beat the ricotta with the eggs and most of the parmesan. Season, then stir in the chard.
Pour into a buttered baking dish about 20cm square, spoon on the tomatoes and sprinkle on the rest of the cheese.
Bake for 10-15 minutes until just set in the middle. Grill for 2 minutes until golden if you want more colour. Serve with a crisp green salad.