In a small bowl, mix the cheese, minced garlic, lemon juice, and oregano with a fork until smooth.
Place chicken in a large heavy-duty freezer bag. Pound the chicken to 1/2-inch thickness. Remove chicken and discard bag.
With a knife or a small metal spatula, spread the cheese mixture over the inside of each chicken breast half to within 1/2 inch of the edge. Fold the chicken breast in half crosswise to enclose filling and secure with a wooden toothpick.
On a plate, mix the salt, garlic powder, pepper and 1 TB flour and use this to coat the chicken.
Heat oil on medium-high heat in a 12-inch skillet (it doesn't have to be nonstick). Add chicken to the hot oil and cook until golden brown on both sides, turning only once.
While chicken is cooking, mix the chicken broth and remaining flour in a small bowl or measuring cup until smooth.
Add the broth mixture, tomato and spinach to skillet with the chicken. Heat to boiling over high heat. Lower heat and simmer, covered, for 8-10 minutes until chicken is tender and no longer pink in the center.
Remove toothpicks and discard.