Season and sear the chicken: Season the chicken breasts generously with salt and black pepper. Heat the olive oil in a large skillet or pot over medium-high heat. Once hot, add the chicken breasts and sear for 4-5 minutes per side until golden brown. Remove the chicken and set aside.
Sauté the aromatics: In the same skillet, add the finely chopped onion. Cook for 3-4 minutes until softened and translucent. Add the minced garlic and cook for another 30 seconds until fragrant.
Combine marinara and chicken: Pour in the water and marinara sauce, and stir to combine with the onions and garlic. Return the seared chicken breasts to the skillet, nestling them into the sauce.
Build the sauce: Add the Kalamata olives, drained capers, anchovy fillets, sweet paprika, and dried oregano. Mix well, breaking up the anchovies with a spoon so they dissolve into the sauce.
Simmer the chicken: Reduce the heat to low, cover, and let it simmer for 25-30 minutes, or until the chicken is fully cooked and tender.
Finish with lemon juice: If using, stir in the lemon juice just before serving. This adds a citric note to balance the flavors.
Serve: Plate the chicken breasts and spoon the sauce generously over the top. Serve with a side of steamed vegetables or a light salad for a complete meal.