Preheat oven to 350°F. Rinse chicken breasts and pat dry. Cover with plastic wrap and pound flat. Remove wrap and rub both sides of each breast with one tablespoon olive oil and set aside. Sauté spinach, onions, and mushrooms in a sauté pan over medium heat with two tablespoons olive oil and remove from heat. Allow mixture to cool slightly before stirring in feta cheese. Set aside. Sauté chopped tomatoes in a sauté pan over medium heat with one table spoon olive oil. Set aside. Season chicken with thyme, rosemary, and sea salt. Divide spinach mixture into thirds, and set one-third aside. Divide the two-thirds portion into four, and spread one fourth on each chicken breast, roll, then secure with toothpick at the center. Repeat for each breast. Bake for 20 minutes. Top each serving with remaining spinach mixture (about two tablespoons each) and garnish with cooked tomatoes and chopped chives.
