Serves 4; 1 chicken breast and ½ cup apple-spinach mixture per serving)
2 teaspoons vegetable oil
4 (4-ounce) skinless, boneless chicken breast halves
¼ teaspoon salt
¼ teaspoon black pepper
1 garlic clove minced (about ½ teaspoon)
1 tablespoon spicy brown mustard
½ cup apple cider
1 medium Granny Smith apple, cored and sliced
6 cups fresh baby spinach or chopped Swiss chard
Heat oil in a large nonstick skillet over medium-high heat.  Sprinkle chicken with salt and pepper, and add to skillet.  Cook 5-6 minutes on each side or until well browned.  Remove to plate, and keep warm. 
Add next 4 ingredients (through apple slices) to pan; bring to a boil.  Reduce heat, simmer, stirring often, 5 minutes.  Return chicken and juices to pan.  Cook, stirring occasionally, until sauce thickens and the chicken is cooked through, about 3 minutes. 
Add spinach to pan, and toss until wilted, about 1 minute.  Serve hot.