Preheat oven to 425° F.

Heat 1 tablespoon olive oil in a large skillet over medium high heat. Sauté the onions until soft and translucent, about 5 minutes. Add the garlic and cook for another minute. Squeeze all the liquid from the spinach and add to the onions. Add 1 teaspoon salt (if desired), ½ teaspoon pepper, the oregano and the lemon zest. Raise the heat to high and cook until all the liquid is gone, about 1 minute. Take off the heat and let cool while preparing the chicken.

Butterfly the chicken breasts by placing each one flat on a cutting board. Place one hand on top and with the edge of a sharp knife parallel to the cutting board, begin cutting down the length of the side of the breast. Carefully slice the breast in half widthwise almost to the other edge. Open up the chicken breast like a book.

Add the feta if using to the spinach mixture and stir to combine. Stuff each chicken breast with one fourth of the stuffing, letting it overflow at the edges. Drizzle 1 tablespoon of olive oil in a rimmed baking dish and place the stuffed breasts in the baking dish. Squeeze the lemon juice over and top with the chopped pistachios. Top the pistachios with a pinch of salt (if desired) and pepper and drizzle with 1 tablespoon of olive oil.

Place the halved tomatoes in another baking dish and drizzle with the remaining tablespoon of olive oil. Add 1 teaspoon salt and ½ teaspoon pepper. Toss to coat.

Put both baking dishes in the oven and roast for 20 minutes or until the juices of the chicken run clear. Top the chicken breasts with the roasted tomatoes.
 

Nutritional Facts (Amount per Serving):

Calories 480, Total Fat 32g, Saturated Fat 7g, Monounsaturated Fat 17g, Polyunsaturated Fat 5g, Cholesterol 85mg, Sodium 540mg, Potassium 1035mg, Magnesium 113mg, Total Carbohydrate 16g, Dietary Fiber 6g, Sugars 5g, Protein 37g