Set a large skillet over medium-high heat with the oil. Once hot, add the onion and garlic and cook until softened, about 3 minutes. Stir in the red bell pepper and cook until tender, about 20 minutes. Add the cumin, paprika, cayenne, and harissa and cook 1 to 2 minutes longer. Pour in tomatoes and break up the large tomatoes with a wooden spoon. Season with the salt and pepper, and simmer until the tomatoes have thickened, about 15 minutes. Remove from the heat.