For the masala sauce: Melt the butter in a small saucepan and add the mustard seeds. Once they start to crackle, add the garlic paste and stir through. Add the spices: paprika, cayenne, turmeric, garam masala and salt. Cook for 1 minute then add in the tomato sauce. Combine, and then take off the heat.
Prepare the potatoes: Wash and peel the potatoes. Chop into thick chips. Rinse under cold water and pat dry.
Heat the oil to 140C. Carefully add half of the potatoes to the hot oil and cook for 6-8 minutes until soft but still pale. Drain on kitchen towel while you repeat with the second batch. Spread them out to cool and heat the oil up to 180C. Once the potatoes have rested for 10 minutes, cook them at the higher temperature until they are golden brown and crisp. Do this in batches if necessary. Drain on kitchen paper.
To serve: Put the masala sauce into a large mixing bowl and add in the hot, drained chips. Mix well, then tip onto a serving plate. Squeeze lemon wedges over the masala fries and sprinkle finely sliced chilli and coriander on top, then serve.