For the coconut dal, heat the oil in a large non-stick saucepan over a low heat. Add the onion and garlic and fry very gently for 5-6 minutes, stirring regularly until softened and just beginning to brown. Stir in the curry powder and cook for a further 30-40 seconds.
Add the lentils, coconut milk, 400ml/14fl oz water and half a teaspoon of salt to the onion and garlic mixture, then bring it to the boil. Reduce the heat until the mixture is simmering. Simmer, stirring regularly, for 25-30 minutes, or until the lentils are swollen and tender and the dal has thickened. Stir in the lime or lemon juice, season, to taste, with the remaining salt and plenty of freshly ground black pepper, then remove from the heat and set aside until needed. (The dal can be reheated when required; meanwhile, the flavours will intensify as it stands.)
For the pakoras, in a large mixing bowl, mix the thawed vegetables with the chopped coriander, if using. Add the flour, salt, chilli powder and turmeric and stir to combine and coat the vegetables in the mixture.
Stir in the 2 tablespoons of sunflower oil using a large wooden spoon, then gradually add 150ml/5fl oz tepid water in a thin stream, stirring constantly, until the ingredients coating the vegetables form a thick, smooth batter.
Add enough sunflower oil to reach 2.5cm/1in up the sides of a deep-sided, heavy-based saucepan. Heat the oil to 160C, or until a breadcrumb sizzles and turns golden-brown when dropped into it. Take care not to allow the oil to overheat. (Caution: Hot oil can be dangerous. Do not leave unattended.)
Carefully lower dessertspoonfuls of the pakora batter into the hot oil, in batches of 6 or 7. You will need to push the batter off the spoon using a second spoon. Cook the pakoras for 2½-3 minutes, turning once during cooking, until they are crisp and golden-brown and float to the surface of the oil. Remove the pakoras from the hot oil using a slotted spoon and set aside to drain on kitchen paper. Keep warm. Repeat the process with the remaining pakora batter.
Just before serving, reheat the dal, adding a little extra boiling water to loosen it if necessary.
To serve, ladle the dal onto plates with a spoonful of yoghurt and mango chutney alongside. Garnish with a lime wedge and chopped coriander, if using. Serve the pakoras on separate side plates, or as a starter.