Add the olive oil to a large saucepan or Dutch oven and set over medium heat. Once shimmering, add the leeks and a generous pinch of salt. Cook, stirring occasionally, until the leeks are very soft and beginning to brown around the edges - about 10 minutes.
Add the garlic, tomato paste and miso paste and cook for another minute or two, just until the tomato paste darkens and begins to stick to the bottom of the pan. Stir in the cumin, za'atar, smoked paprika, coriander, lemon zest and chili flakes. Cook until fragrant - about 30 seconds.
Stir in the chickpeas and lentils. Pour over the stock and bring to a boil. Reduce the heat to a simmer and allow to cook until the lentils are very tender - about 15-20 minutes.
Add 1 tbsp of lemon juice and parsley. Stir and taste to adjust for seasoning, adding salt or more lemon juice where needed. Serve, topping with more parsley if desired.