Peel and finely chop the onions. Warm a large pan over a medium-low heat for 2 mins. Add 2 tbsp oil and the onions. Season with a pinch of salt and pepper. Cook for 8 mins, stirring occasionally, till the onions are soft and slightly browned.
While the onions cook, peel the potatoes and chop them into chunks around 2cm across. Peel the apples, quarter them, slice out the cores then halve each quarter. When the onions have softened, add the apples and potatoes to the pan with 2 tbsp water. Season with a little more salt and pepper. Cook for 8 mins, stirring occasionally.
While the apples and potatoes cook, peel and grate or crush the garlic. Peel and grate the ginger and turmeric (the turmeric will stain things, so it’s best grated onto a plate rather than a wooden or plastic board). Drain the chickpeas and rinse them.
Stir the garlic, ginger and turmeric into the pan. Add 1 tbsp tikka masala spices and 12 curry leaves. Cook and stir for 1 min.
Add the chickpeas and the chopped tomatoes. Pour in 400ml boiling water. Cover and bring to the boil. Once it’s boiling, turn the heat down and simmer for 45 mins till the potatoes are tender. Stir every so often, and add a splash more water if it looks dry.
Taste the curry and add a pinch more seasoning if needed. Ladle into warm bowls and serve.