Caramelised onion tartlets

Preparation

1. Preheat the oven to 230ºC. Lightly brush 12 deep bun tins with a little of the melted butter. Using a 7.5 cm pastry cutter, cut a disc from each slice of bread. Flatten each bread disc with a rolling pin, then press into the buttered bun tins to line them evenly, curving the edge of the bread slightly to make large scallop shapes.
2. Brush the bread cases with the rest of the melted butter and bake for 8–10 minutes, or until crisp and golden. Set aside in a warm place until ready to fill. (If made ahead of time, keep the bread cases in an airtight container.)
3. Heat the oil in a large heavy-based frying pan with a well-fitting lid. Add the onions and stir well. Cover with the lid and cook over a low heat for 20 minutes, or until the onions are very soft.
4. Remove the lid, increase the heat and cook rapidly, stirring, until the onions turn a dark golden brown. Remove from the heat and stir in the sun-dried tomatoes and thyme. Season with salt and pepper to taste. (If made ahead, cool the filling and keep in the fridge, then reheat just before filling the bread cases.)
5. Divide the onion filling among the croustades, then scatter the walnut pieces over the top. Serve hot.