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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.
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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.
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3
Preheat the oven to 375 degrees F (190 degrees C).
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4
Spray a 9x13-inch baking dish with cooking spray.
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5
Lightly toss potatoes, cabbage, corned beef, and Swiss cheese in a large bowl until well combined.
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6
Stir together mustard, white wine, garlic powder, and black pepper in a bowl until smooth; mix into corned beef mixture.
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7
Spread the mixture into the prepared baking dish.
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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.
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9
Repeat three more times, spraying, folding, and respraying the phyllo sheet each time.
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10
Brush the top of last sheet with melted butter.
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11
Bake in the preheated oven until phyllo sheets are crisp and browned and the casserole is hot, 45 to 55 minutes.
