Heat a large skillet on medium high. Drizzle with olive oil and add the onion and leek, season with salt and pepper, and cook, tossing often, until completely softened and starting to stick to the pan and caramelize, about 10 minutes. Add the garlic and stir for 1 minute. Turn off heat.
Preheat oven to 180˚C / 350˚F. Prepare an 8-inch (20 cm) square or circular dish by lining it with baking paper or applying a non-stick spray.
In a large bowl, add the chickpea flour, baking powder, turmeric, saffron (if using), season with about 1 teaspoon of salt and lots of black pepper and whisk to combine. Add the vegetable stock and whisk well to combine, until mostly smooth and lump free. Fold in the onion and leek mix, kale, herbs and walnuts until well combined.
Pour the batter into the prepared pan and bake until slightly golden and firm on top, 25 to 30 minutes. Allow to cool for 5 to 10 minutes.
Meanwhile, make the lemon tahini: place the tahini, garlic, lemon juice and 2 tablespoons of water into a bowl and whisk to combine. If the tahini seizes or is too thick, add a touch more water until it is smooth and pourable. Season well with salt and pepper.
To serve, cut the frittata into squares and top with the lemon tahini, a drizzle of pomegranate molasses (if using) and a final scatter of walnuts.