In a large bowl, combine the self-raising flour, suet, and salt. Gradually add the cold water while mixing with a knife until a firm dough forms. Avoid overworking the pastry to keep it light. Wrap in cling film and chill for 20 minutes before rolling.

Melt butter with oil in a heavy pan over low heat. Add the sliced onions and cook slowly, stirring often, for 25 to 30 minutes until golden and sweet. Sprinkle in the sugar and vinegar, then season with salt and pepper. Allow the mixture to cool before filling.

Lightly flour a clean surface. Roll the chilled dough into a rectangle about 20 cm wide and 30 cm long. Keep the thickness even so the pastry bakes uniformly.

Spoon the cooled caramelised onion mixture lengthways down the centre of the pastry, leaving space around the edges for sealing. Keep the filling compact to avoid leakage during baking.

Bring both long sides of the pastry up over the filling to meet in the middle. Pinch the edges firmly together, then fold in the short ends. Crimp well with your fingers or a fork for a traditional look.

Transfer the sealed clanger onto a lined baking tray. Brush the surface generously with beaten egg for a glossy golden finish.

Place the tray in a preheated oven at 200°C (180°C fan). Bake for 35 to 40 minutes until the pastry is crisp, risen, and richly browned. Avoid opening the oven too soon to maintain an even rise.

Remove from the oven and allow the clanger to cool for at least 10 minutes. This resting period helps the filling settle and makes slicing easier.

Slice into generous portions and serve warm. A crisp green salad or pickled vegetables make excellent accompaniments. For a more rustic touch, serve with a mug of strong English tea.