For the chicken breasts:
2 lbs. boneless, skinless chicken breasts (about 6 breast halves)
1/2 c. extra virgin olive oil
1 tbsp. minced fresh rosemary leaves
1 tbsp. minced fresh sage leaves
4 garlic cloves, crushed
Salt and freshly ground black pepper
For the sauce:
3 tbsp. extra virgin olive oil
8 garlic cloves, crushed
1-1/2 lbs. vine-ripened plum tomatoes, cored, peeled and seeded, juice reserved
Large pinch crushed red pepper
1/2 c. finely shredded fresh basil leaves
Cut each chicken breast half crosswise on the bias into three, more or less, even pieces. Place three of the chicken pieces between two sheets of plastic wrap and pound them lightly with a meat mallet to a thickness of about 1/4-inch. Repeat with remaining chicken pieces.
Stir the olive oil, rosemary, sage and garlic cloves together in a large bowl. Add the chicken pieces and toss to coat well. Season with salt and pepper and toss well again. Cover the bowl and refrigerate the chicken for at least 8 hours, or overnight. Remove the chicken to room temperature about 30 minutes before cooking.
Just before serving, make the sauce. In a medium-size saucepan, heat the olive oil over medium heat. Add the garlic and cook, shaking the pan occasionally, until it is light brown, about 2 minutes. Remove the pan from the heat and carefully add the tomatoes and their reserved juice. Return to the heat. Stir in the crushed red pepper, season with salt, and adjust the level of heat to a simmer. Simmer until the tomatoes are softened and the sauce is pink and thick, about 5 minutes. Stir in 1/4 c. of the basil leaves and keep the sauce warm, covered, off the heat.
Heat one or two large, nonstick or well-seasoned cast-iron skillets over medium-high heat. Add as many of the chicken pieces as will fit in a single layer. Cook until well browned on the underside, about 2 minutes. Turn the chicken and cook until the second side is browned and no trace of pink remains in the center, 1 to 2 minutes. If necessary, keep the first batch of chicken warm on a baking sheet in an oven turned to the lowest setting and cook the remaining chicken in batches.
Divide the chicken pieces among serving plates, overlapping them in a circular pattern in the center of the plates. Spoon the sauce into the center of each circle, drizzling some of it over the chicken pieces. Sprinkle with the remaining 1/4 c. shredded basil and serve.