Sprinkle each side of chicken with Italian seasoning, salt and pepper. Set aside. In a large skillet, over medium high heat, melt butter with olive oil. Add onion and garlic to skillet and saute for about 2 minutes.
Move onions and garlic to the side of the pan and add seasoned chicken to skillet. Cook chicken for 2 to 3 minutes, on each side, until nicely browned. Stir chicken, onions and garlic together then carefully transfer to the crock pot. Pour in chicken broth.
Cover and cook on low heat for 3 to 4 hours OR on high heat for 2 to 3 hours. (Internal temperature of chicken needs to reach 165 degrees F.) Remove chicken and cover to keep warm.
Add heavy cream, mustard and lemon juice to crock pot and whisk. To thicken sauce whisk in 1 tablespoon cornstarch. Place chicken back in the crock pot. Cover with lid and cook another 30 minutes or until sauce thickens a little. Uncover and stir. Add additional salt and pepper to taste.
Serve chicken with creamy lemon sauce and enjoy!