1. Preheat oven to 400°.
2. Cook guanciale in a large ovenproof skillet over medium heat 10 minutes or until crisp, stirring frequently; remove from pan using a slotted spoon. Combine guanciale, 1 tablespoon drippings, cheese, tomatoes, and 1/4 teaspoon salt in a small bowl, stirring well.
3. Cut a horizontal slit through thickest portion of each chicken breast half to form a pocket. Stuff 2 tablespoons cheese mixture into each pocket; secure with a wooden pick. Sprinkle both sides of chicken evenly with remaining 1/4 teaspoon salt and pepper.
4. Wipe pan clean with paper towels. Return pan to medium-high heat. Add oil to pan; swirl to coat. Add chicken; sauté 4 minutes. Turn chicken over. Place pan in oven; bake at 400° for 8 minutes or until chicken is done. Let stand 5 minutes. Discard wooden picks. Cut chicken diagonally into 1/2-inch-thick slices, if desired. Serve with pan juices.