To make the dough, mix the flour and salt together in a bowl, then add the water and oil and bring together to make a dough. Transfer to a floured work surface and knead well. Roll the dough into two thin rectangles, each measuring around 25 centimeters x 28 centimeters/10 inches x 11 inches. [Do the same with the puff pastry, if using that.]

To make the topping, dry fry the bacon lardons in a frying pan until the fat renders out and they are golden brown. Drain on kitchen paper.

Preheat the oven to 450°F/230°C/Gas 8 and line two large flat baking trays with baking paper that can be used at high heat. Place the dough rectangles on the trays.

Spread the crème fraîche over the dough leaving a little border around the edges. Scatter over the onion, dot with the lardons, and sprinkle liberally with Emmental. Season with salt, pepper, and a grating of nutmeg.

Bake for 10–12 minutes, or until the base is crisp and the cheese is bubbling. Slide the tarts onto a wooden board and use a pizza cutter to cut them into portions. Serve immediately with drinks or as a light lunch with a green salad, dressed with olive oil.