Place half the sauerkraut in a 3 1/2 quart electric slow cooker. Drizzle on about 1/3 cup of the dressing. Top with 3 chicken breast halves and spread the mustard over the chicken. Top with the remaining sauerkraut and chicken breasts. Drizzle another 1/3 cup dressing over the casserole. Refrigerate the remaining dressing until serving them. Cover and cook on the low heat setting about 3 1/2 to 4 hours, or until the chicken is white throughout and tender. To serve, spoon the casserole onto 4 or 5 plates. Sprinkle 3 to 4 tablespoons of cheese over and drizzle about 1 tablespoon of the Russian dressing on top. Serve immediately, garnished with parsley.