Heat olive oil in a frying pan over medium heat. Add mustard seeds and cumin seeds; when sizzling, stir in onions. Cook for 10 minutes until caramelized. Add apple cider vinegar, potatoes, and 200ml water. Cook for 15 minutes until potatoes are soft.

Boil cauliflower florets in a large pot for 8–10 minutes until tender. Drain and set aside.

Melt the butter in a saucepan over medium heat. Add the garlic, ginger, chili powder, coriander, and cumin, stirring continuously, and cook for 2 minutes. Sprinkle in the flour and cook for another 2 minutes. Gradually pour in the milk, whisking constantly until the mixture thickens. Stir in the cheese and 1 teaspoon of salt until fully combined. Gently fold in the cauliflower, ensuring it is evenly coated.

Heat the oil in a separate pan over medium heat. Add the leeks and ½ teaspoon of salt, cooking for 5 minutes until softened. Stir in the garlic and cook for an additional 2 minutes. Sprinkle in the coriander and mix thoroughly.

Allow all the fillings to cool completely.

Preheat the oven to 180°C. Cut a large circle out of one puff pastry sheet and place it on a baking tray. Use a fork to prick the center, leaving a 3cm border untouched. Brush the border with egg wash.

Layer the fillings on the pastry: start with the onion-potato mixture, followed by the cauliflower cheese (avoiding any excess sauce), and finish with the leeks.

Place the second pastry sheet over the fillings, pressing the edges together to seal, and trim any excess. Score the top for decoration, then brush with egg wash.

Bake for 35–40 minutes until golden and crisp. Allow to cool slightly, then serve with the reserved sauce if desired.