Put the oil into a large sauté pan with a lid and place over medium-high heat. Once hot, add the onion and fry for 8 minutes, stirring every once in a while, until soft and caramelized. Add the harissa, tomatoes, olives, capers, and 1/2 teaspoon salt and continue to fry for 3–4 minutes, stirring frequently, until the tomatoes start to break down. Add the water and stir to mix thoroughly. Once boiling, decrease the heat to medium-low, cover the pan, and simmer for 10 minutes. Remove the lid and continue to cook for 4–5 minutes, until the sauce is thick and rich. Stir in 1/2 cup/10 grams of the parsley and set aside.
Meanwhile, fill a large pot with plenty of salted water and place over high heat. Once boiling, add the pappardelle and cook according to the package instructions, until al dente. Drain well.
Return the pasta to the pot along with the harissa sauce and 1∕8 teaspoon of salt. Mix together well, then divide among 4 shallow bowls. Serve hot, with a spoonful of yogurt and a final sprinkle of parsley.