You'll find this simple onion tart everywhere in Alsace, especially at bakeries and épiceries where they're a favourite counter snack. They're best eaten warm, but are equally good cold at a picnic.
Process flour, butter and 1 tsp salt in a food processor until mixture resembles breadcrumbs. Add egg, then while pulsing gradually add 2-3 tbsp iced water until pastry forms into a ball. Transfer to a floured surface, knead for 30 seconds, wrap in plastic wrap and refrigerate for 30 minutes.
Preheat oven to 200C. Roll out pastry to 5mm thick on a floured surface, cut out four 16cm-diameter rounds and use to line four 12cm-diameter tart tins, trimming excess pastry with a knife. Place tins in freezer for 10 minutes, then bake for 12 minutes or until golden. Set aside until ready to use.
For onion filling, melt butter in a large saucepan over low heat, add onions and cook, stirring occasionally, for about 30 minutes or until softened but not brown. Transfer to a bowl and cool. Whisk eggs, crème fraîche and nutmeg together and season with sea salt and freshly ground black pepper. Add to onions and combine well. Divide filling among tart tins and bake for 15 minutes or until golden. Serve warm, scattered with chervil and goat's cheese to the side.