Add another tablespoon of oil to the skillet. Saute the onion and peppers over medium-high heat until soft, about 8 minutes. Stir in the garlic, cumin, and harissa, if using. Cook until fragrant, 2 minutes. Carefully pour in the tomatoes. Bring to a simmer over high heat. Cook until the tomatoes have begun releasing their liquid, 5 minutes. Using a fork or masher, crush the tomatoes to break up the larger chunks and create a saucy consistency. Reduce the heat to medium-low and simmer, stirring occasionally, until the sauce has thickened and the tomatoes have lost their acidity, about 20 - 30 minutes.