Preheat oven to 350°. Cut chicken breasts in half crosswise. Pound with a meat mallet to 1/2-in. thickness; sprinkle with salt and pepper. Place flour in a shallow bowl. Dip chicken in flour to coat both sides; shake off excess., In a large skillet, heat 1 tablespoon oil over medium-high heat. Brown chicken in batches, adding additional oil as needed. Transfer chicken to an ungreased 13x9-in. baking dish., Add stock, capers and lemon juice to pan, stirring to loosen browned bits from pan. Whisk in butter, 1 tablespoon at a time, until creamy. Pour sauce over chicken. Bake 5-10 minutes or until no longer pink.