For the vegetable samosas, preheat a deep-fat fryer to 180C/350F. Or half-fill a deep, heavy-based pan with vegetable oil and heat until a breadcrumb dropped in sizzles and turns golden-brown in 30 seconds. (Caution: hot oil can be dangerous. Do not leave unattended.)
For the pastry, mix the plain flour, self-raising flour and butter in a bowl. Add a splash of warm water and knead well to make an elastic dough.
Divide the dough into 12 small balls and roll each out into a circle. Cut each into two half-circles. You should have 24 half-circles.
For the filling, heat the vegetable oil in a frying pan. Add the onion and fry until golden-brown. Add the ginger, spices and tomato purée and cook for 2-3 minutes. Add the potatoes and peas and season with salt. Cook the mixture over a low heat, stirring regularly, until the mixture is soft. Finish with the chopped fresh coriander.
Add a teaspoon of the filling to the middle of one semi-circle and fold into a triangle. Repeat to use up all the dough and filling.
Deep fry the samosas in the hot oil or deep-fat fryer. Fry in small batches for 4-5 minutes, or until golden-brown and crisp. Remove from the oil with a slotted spoon and drain on kitchen paper.
For the chilli sauce, heat the sugar in a heavy-based saucepan over a medium heat until it melts and forms a golden-brown caramel.
Meanwhile, blend all of the remaining chilli sauce ingredients, except the crème fraîche, in a food processor to a smooth purée.
Once the sugar has caramelised, add the puréed mixture to the caramel and stir well. Bring the mixture to the boil, then reduce the heat and simmer for 4-5 minutes, or until sticky and jam-like. Remove from the heat and set aside to cool.
Serve the samosas on a plate with a bowl of chilli sauce and a spoonful of crème fraîche.