Place chicken breast under plastic wrap and flatten using mallet, rolling pin, or a glass bottle. Season breasts with salt and pepper. In heated skillet add olive oil and butter. Add chicken and cook over a medium heat until golden brown on both sides. Remove chicken to a plate and cover with foil to keep warm. Add scallions and garlic to same skillet. Saute quickly and add white wine. Boil rapidly and reduce to syrup. Add cream and mustard, stir. Return chicken breast to pan and simmer gently until just heated through.