Prepare the Sauce: Heat the olive oil in a large skillet over medium heat. Add the chopped onion and diced red bell pepper and cook until softened, about 5-7 minutes. Add the minced garlic, cumin, paprika, and cayenne (if using) and cook for another 1-2 minutes, until fragrant.
Simmer the Tomatoes: Add the crushed tomatoes and water to the skillet, stirring to combine. Season with salt and pepper to taste. Reduce the heat to low and let the sauce simmer for 15-20 minutes, stirring occasionally, until it has thickened slightly.
Add the Eggs: Use the back of a spoon to make six small wells in the sauce. Crack an egg into each well, being careful not to break the yolks. Cover the skillet with a lid and cook for 5-8 minutes, until the egg whites are set but the yolks are still runny.
Add Feta and Sumac: Once the eggs are cooked to your liking, sprinkle the crumbled feta cheese over the top of the dish. Remove the skillet from the heat and sprinkle the sumac over the eggs and sauce.
Garnish and Serve: Garnish the shakshuka with fresh parsley or cilantro and serve directly from the skillet. Accompany the dish with warm pita bread or crusty bread for dipping.