Step 1In a 6-quart, heavy-bottom sauce pot, heat the olive oil over medium-high heat. Stir in the onion and cook until softened, about 5 minutes, stirring frequently. Stir in the ginger, garlic and lemon zest and continue to cook until aromatic, about 2 minutes.
Step 2Stir in the cinnamon, cumin, paprika, turmeric and cardamom and cook, stirring constantly to gently toast the spices, about 1 minute. Add the tomato paste and continue to stir until it darkens slightly, coats the onion and leaves a thin film on the bottom of the pan, about 1 minute, being careful not to scorch the bottom of the pan.
Step 3Stir in the lemon juice and diced tomatoes, scraping the bottom of the pot with a spatula or wooden spoon to lift the tomato paste. Stir in the crumbled sausage.
Step 4Add 8 cups chicken broth and stir in 1 teaspoon salt. Stir in the lentils, saffron, if using, and harissa. Bring the mixture to a boil over high heat, stirring frequently. Reduce the heat to a gentle simmer.
Step 5Cook the chili, uncovered, until the lentils are tender but firm, about 20 to 25 minutes, stirring occasionally and adding additional chicken broth as needed, 1 cup at a time, to keep the lentils barely submerged at all times. The amount of liquid will vary depending on the lentils, though no more than 2 additional cups should be necessary.
Step 6Stir in the minced parsley and taste the chili, adjusting the seasoning and heat as desired with additional salt and harissa. Continue to cook until the lentils are very tender, stirring occasionally, another 15 minutes or so. The lentils will absorb more liquid and the chili will thicken; make sure to stir the bottom of the pan to watch and prevent the chili from burning at the bottom, and adjust the thickness as desired with additional broth. Check the seasoning once more, adding additional harissa, salt or lemon juice as desired.
Step 7Serve the chili with chopped cilantro, crumbled feta cheese and additional harissa. The chili will keep for up to 5 days, covered and refrigerated.