Preheat a deep-fat fryer or a pan with vegetable oil to 180°C.
Mix plain flour, self-raising flour, and butter, then knead with warm water into an elastic dough.
Divide dough into 12 balls, roll each into a circle, and cut into 24 half-circles.
Fry onion, ginger, spices, and tomato purée for 3 minutes before adding potatoes and peas.
Cook until soft, stir in fresh coriander, and let the mixture cool slightly.
Place filling in the middle of each dough semi-circle and fold into triangles.
Deep fry samosas in batches for 5 minutes until golden and drain on paper.
Melt sugar in a saucepan until it forms a golden-brown caramel.
Blend sauce ingredients into a smooth purée and stir into the hot caramel.
Boil and simmer sauce for 5 minutes until sticky, then cool and serve with samosas and crème fraîche.