1Melt butter in 12-inch nonstick skillet over medium heat. Cook leek in butter 2 minutes, stirring occasionally. Stir in mushrooms. Cook 2 minutes, stirring occasionally. Remove mushroom mixture from skillet.
2Sprinkle both sides of chicken with seasoned salt and pepper. Cook chicken in skillet 3 to 5 minutes or until browned on both sides. Layer chicken and mushroom mixture in 5- to 6-quart slow cooker. Mix broth, wine and mustard; pour over chicken and mushrooms.
3Cover and cook on low heat setting 4 to 5 hours or until juice of chicken is no longer pink when centers of thickest pieces are cut.
4Remove chicken from cooker; cover to keep warm. Mix half-and-half and flour until smooth; stir into mixture in cooker. Cover and cook on high heat setting 10 to 15 minutes, stirring occasionally, until mixture thickens. Stir in bell peppers. Serve sauce with chicken.