Preheat the oven to 420F. 

Bring a large pot of water to the boil. Add 2 tablespoons of salt into the water. 

Meanwhile, roast the chicken: Spread the sliced onions onto a baking tray and place the chicken pieces on top. Rub about 1 tablespoons of olive oil onto all the chicken pieces. Sprinkle the chicken pieces with 1 tablespoon of salt, 1 teaspoon pepper, 2 teaspoons of cinnamon, ½ teaspoon of allspice and ½ teaspoon of paprika. Rub the spiced onto all surfaces of the chicken pieces. Place the chicken pieces with the skin side up. Transfer the tray into the oven and roast the chicken for about 25 to 30 minutes, until golden and cooked through. Remove the chicken from the oven and set aside. Lower the oven temperature to 350F. 

Make the sauce: Heat 2 tablespoons of olive oil in a small pot over medium low heat and add the garlic. Cook until the garlic is softened, about 1 to 2 minutes. Add the tomato paste and stir until the tomato paste becomes silky and begins to bubble slightly in the pot, about 1 more minute. Add the sugar, ¼ teaspoon cinnamon, ¼ teaspoon allspice, ½ teaspoon paprika, ¼ teaspoon of salt and ½ teaspoon of pepper. Mix well. Add 1 ¾ cups of water and stir well. Increase the heat to medium high. Bring the mixture to a boil and reduce the heat to medium low. Continue cooking the sauce on a simmer for about 10 more minutes or until the sauce has thickened. Set aside. 

Place the spaghetti into the pot of boiling water. Cook the spaghetti for 2 minutes less than the packet directions. Reserve 3 tablespoons of hot pasta water on the side. Drain the spaghetti.

Place the cooked spaghetti, spiced tomato sauce and the roasted onions into a large baking dish (about 9 inches by 13 inches). Toss the spaghetti until it is evenly coated with the sauce and onions. If the sauce is not coating easily, add the reserved pasta water and continue to toss. Nestle the chicken pieces over the spaghetti with a bit of space between each piece. Sprinkle the top of the spaghetti and chicken with ¼ teaspoon salt, ¼ teaspoon pepper and ¼ teaspoon cinnamon. Transfer the baking dish into the oven and bake for about 30 to 40 minutes or until the spaghetti and chicken are crispy and golden. 

Serve immediately.