Prepare the saffron water, if using: In a kettle or a saucepan, bring 2 tablespoons of water to a boil, then let stand for 2 minutes to allow the temperature to drop slightly while you grind the saffron. Using a mortar and pestle, grind the saffron to a fine powder. Add the hot water, gently stir, cover and let steep until ready to use.

In a large (12-inch) deep pan with a lid, or large (5-quart) Dutch oven, heat the oil over medium. Add the onion, and cook, stirring occasionally, until golden, 10 to 12 minutes. Sprinkle with a little salt, add the turmeric and pepper, and stir until fragrant, about 30 seconds more.

Add the chicken, season with 2 teaspoons of salt, and cook, stirring occasionally, until the chicken is no longer pink, 3 to 5 minutes. Add the tomato paste, and cook, stirring frequently, until the tomato paste is no longer raw and infuses its color into the oil, about 2 minutes. Drizzle in a little more oil if the pan is too dry.

Add the green beans, carrots, potatoes and cinnamon stick, and season with 1 teaspoon salt. Cook, stirring occasionally, until the vegetables just come out of their raw state, about 3 minutes. Add 1 cup of water. Increase to medium-high, bring to a boil, cover, adjust heat to low and gently simmer, stirring occasionally, until the chicken is tender and the vegetables are fork-tender but not mushy, 30 to 35 minutes.

Remove the cinnamon stick from the pot, add the lemon juice and saffron water, stir and simmer, with the pot partially covered, until the flavors come to life, about 15 minutes. While the stew simmers, taste for salt and lemon juice and adjust as needed.