Preheat oven to 400°.
. Squeeze 1 lemon half, and reserve juice. Thinly slice other lemon half crosswise, and quarter each slice. Combine quartered lemon slices, tomatoes, and next 3 ingredients in a medium bowl; add 2 teaspoons olive oil, and toss to combine.
. Sprinkle both sides of chicken evenly with Greek seasoning. Heat remaining 1 tablespoon oil in a large ovenproof skillet over medium-high heat. Add chicken, and cook 2 to 3 minutes on each side or until golden brown. Remove chicken from pan; keep warm.
. Add reserved lemon juice to pan, scraping pan to loosen browned bits. Add tomato mixture to pan, stirring gently.
. Place chicken on top of tomato mixture, and place pan in oven. Bake at 400° for 20 to 22 minutes or until chicken is done. Garnish with oregano sprigs, if desired.