Begin by marinating the chicken breasts in a mixture of red wine vinegar, salt, pepper, and garlic powder. Allow the chicken to marinate for at least 30 minutes in the refrigerator.
Preheat the oven to 375°F (190°C).
Heat a large skillet or oven-safe pan over high heat. Add olive oil to the skillet. Once the skillet is hot, add the chicken breasts and sear on each side until golden brown, about 2-3 minutes per side.
Transfer the seared chicken breasts to a baking dish or roasting pan. Sprinkle oregano, salt, and pepper over the chicken.
Pour chicken broth into a bowl and dissolve a packet of stevia in the broth. Then, pour the stevia-infused chicken broth, sherry vinegar, and red wine vinegar over the chicken. Scatter green olives and pitted prunes around the chicken in the baking dish.
Cover the baking dish with aluminum foil and place it in the preheated oven. Bake the chicken for about 25-30 minutes, or until the internal temperature reaches 165°F (74°C) and the chicken is cooked through.
Once done, remove the chicken from the oven and let it rest for a few minutes before serving. Serve the Marbella chicken hot, garnished with additional olives and prunes if desired.
For the sauce, if desired, strain the cooking sauce to remove any impurities and transfer it to a small saucepan. Dissolve a teaspoon of cornstarch in a quarter cup of water, then add it to the saucepan. Cook the sauce over medium heat, stirring continuously, until it thickens to your desired consistency. Drizzle this sauce over the chicken before serving. While not essential, this step enhances the dish's overall flavor and presentation.