Preheat oven to 425 F, line 2 baking sheets with parchment paper.

Slice potatoes into ¼" rounds, place in a large bowl.

Drizzle potatoes with the vegetable oil, season with salt and pepper, and gently toss to coat.

Spread potato slices out in a single layer on the baking sheet. Bake in the preheated oven for 25 minutes.

As the potato slices are baking, I like to prepare the topping ingredients - slice the cabbage and green onions, chop up the corned beef, etc, as well as shred the cheese for the sauce. Set aside.

In a small bowl, whisk together the sour cream with 1 tablespoon of horseradish. Taste, add more horseradish, if desired, then season with salt and pepper.

Once the 25 minutes are up, flip each potato slice and bake for another 25 minutes.

As the potatoes bake for the second time, make your Guinness cheese sauce:

In a medium bowl, toss the shredded cheese with the cornstarch, evenly coating the cheese. Set aside.

In a small saucepan, whisk together the beer, milk, and mustard. Heat over medium-high heat until simmering.

Add the cheese, a handful or so at a time, stirring to combine and melt.

Once all cheese is melted and smooth, season with salt and pepper, to taste. Keep warm until the potato slices are finished.

Arrange about half of the potato slices on your serving platter. Scatter about half the cabbage, then drizzle with half the sauce. Top with about half the corned beef.

Repeat with a second layer, using up the remaining potato slices, cabbage, sauce, and corned beef.

Top with sour cream, green onions, and a light sprinkling of caraway seeds, if desired.

Serve immediately.