Make the garlicky yoghurt sauce: transfer the Greek yoghurt to a small bowl, mince the garlic into the bowl, stir to combine well, taste, and adjust as you like, adding another clove of garlic and perhaps some salt. Cover and refrigerate.
Cook the onion: to a large pan over medium-high heat, add the vegetable oil, heat until shimmering, add the diced onion, and fry, stirring occasionally, until the onion is translucent, then reduce to medium heat.
Add green chilli (if using): stir to combine with the onion and continue to cook until the onion begins to brown. If needed, add a little more oil.
Add the spices: while the onion is cooking, add the spices to a small bowl and stir to combine. When the onion begins to brown, reduce the heat to low, add the spice mix, and stir to combine until fragrant.
Add the tomato and chickpeas: stir in the diced tomato and chickpeas, add 2 cups water, increase the heat to medium-high, bring to a boil, then reduce the heat to low.
Simmer and reduce: pop the lid on the pan partially and simmer, stirring occasionally for 20 minutes or so until the chickpeas are soft and sauce has reduced and thickened into a stew; if too thick, add a little more water, stir, and continue to simmer.
Finish the stew: when ready to serve, remove from the heat, stir in 2 tablespoons of garlicky yoghurt, taste, and adjust to suit your palate, adding salt if needed.
Plate and garnish: when ready to serve, distribute between plates, or, if serving at the centre of the table, scoop some steamed basmati rice into a big bowl, ladle the stew on top, drizzle with garlicky yoghurt, and shower with fresh coriander and chilli slices.
Serve: transfer the rest of the garlicky yoghurt to a bowl, sprinkle some dried mint on top, and serve with a classic Afghan salad and warmed naan or other flatbreads for scooping up the stew.