Heat three tablespoons of olive oil in a large pot over medium-high heat. Add the cubed eggplant and ½ teaspoon salt and cook, stirring occasionally, until softened and browned on all sides, 5-6 minutes. Remove the eggplant to a plate and set aside.
Add the remaining tablespoon of oil to the same pot and heat over medium. Add the chopped onion and remaining ¼ teaspoon salt and cook, stirring occasionally, until softened, 3-4 minutes. Add the garlic, harissa, and tomato paste, and stir until fragrant, 1 minute more, then add the diced tomatoes, chickpeas and water.
Bring the stew to a simmer and cook until slightly thickened, about 10 minutes. Stir in the eggplant and heat through, then stir in the lemon juice.
Serve topped with yogurt and sprinkled with parsley.
Tip: Cook the eggplant separately from the stew to retain a firmer texture, and then stir into the pot before serving. Storage: Store in a sealed container in the fridge for up to 3-4 days. Reheat in a pot or the stove or the microwave until heated through.