Step 1
Place red potato cubes into a large saucepan cover with water and bring to a boil. Cook potatoes over medium-low heat until tender about 15 minutes; drain well.
Step 2
Place chopped cabbage into a saucepan and cover with water; bring to a boil. Reduce heat to medium-low and simmer the cabbage until tender and softened about 15 minutes; drain well.
Step 3
Preheat the oven to 375 degrees F (190 degrees C).
Step 4
Spray a 9x13-inch baking dish with cooking spray.
Step 5
Lightly toss potatoes cabbage corned beef and Swiss cheese in a large bowl until well combined.
Step 6
Stir together mustard white wine garlic powder and black pepper in a bowl until smooth; mix into corned beef mixture.
Step 7
Spread the mixture into the prepared baking dish.
Step 8
Lay a phyllo sheet onto a work surface and spray with cooking spray. Place the phyllo sheet onto the casserole folding the overhang back over the dish; spray the dry side of the overhang with more cooking spray.
Step 9
Repeat three more times spraying folding and respraying the phyllo sheet each time.
Step 9
Brush the top of last sheet with melted butter.