Heat oil in a large frying pan over medium-high heat. Add onion. Cook, stirring, for 3 minutes or until onion has softened. Add potato and pumpkin. Cook for 10 minutes or until potato and pumpkin are just tender. Add curry paste and 1 1/2 tablespoons cold water. Cook, stirring, for 1 minute or until fragrant. Stir in peas. Remove from heat.
Preheat oven to 180°C/160°C fan-forced. Line 2 large baking trays with baking paper. Place 1 sheet filo on a flat surface. Brush with butter. Top with 1 sheet filo. Repeat with 1 sheet filo. Cut into 3 long strips. Cover remaining pastry with a damp tea towel so it doesn't dry out.
Place 1 tablespoon potato mixture in 1 corner of pastry. Fold over diagonally to form a triangle. Continue folding, retaining triangle shape. Place on prepared tray. Repeat with remaining filo, butter and potato mixture. Brush with remaining butter. Sprinkle with poppy seeds. Bake for 25 minutes or until golden and crisp. Combine coriander leaves and yoghurt in a small bowl and serve with samosas.