In a sauté pan over medium heat, sauté onion in olive oil until soft and translucent, about 5 minutes. Add both cans of tomatoes, and continue to cook for 10 minutes. Add cilantro, cumin, sugar, and cayenne pepper. Stir gently and cook until cilantro has wilted into the mixture. Season with salt and pepper. Continue to cook for 15 minutes until liquid is reduced down, concentrating the flavors. Adjust seasoning if needed.