Sift the flour into a bowl and make a well in the centre. Whisk milk, water, eggs and salt together until the dough is smooth and forms bubbles. Rest for 15 minutes.

After resting, stir the 100 g of grated Gruyère into the dough.

Slice the onions into rings and fry slowly in butter until deep golden and crisp. Keep warm.

Bring plenty of water to the boil and salt it well. Have a bowl of cold water and a slotted spoon ready. Push the dough into the boiling water in batches using a spaetzle grater, board or press. As soon as the spaetzle float, lift them into the cold water.

Preheat the oven to 180 °C / 350 °F. Drain the spaetzle well and layer them in a buttered dish with the remaining Gruyère and the onions. Bake for 15–20 minutes until golden on top.

Serve immediately with apple sauce on the side.