Toss potatoes with olive oil. Bake potato slices on a baking sheet for about 15 minutes, turn over and bake another 15 - 20 minutes until golden and slightly crisp.
Next, overlap the potatoes in a cast-iron skillet that can be baked in the oven. Season potatoes with salt and pepper. Sprinkle with cheese and corned beef. Bake for 5 minutes to melt cheese. Then sprinkle with scallions and cilantro.
In a separate dish, stir ketchup (and jalapeño peppers or poblano peppers if you like it hot) into the mayo. Serve on the side as a dip.
No oven-safe cast-iron skillet? You can use another oven safe dish like a pie plate or casserole dish.
If you don't have fresh corned beef, go with corned beef from your deli counter.
I recommend aged cheese that you grate yourself. Just give it a quick spin in the food processor. The pre-grated type contains preservatives meant to keep it from clumping. It doesn’t melt quite as well.
Today I used aged cheddar cheese. You could go with swiss cheese (like a classic Reuben sandwich) or provolone or any cheese or combination of cheeses you prefer.
I like to watch out for hidden sugar so I always recommend sugar-free avocado oil mayonnaise from Primal Kitchen. Primal Kitchen has sugar free ketchup too.
Leftovers will keep in an airtight container in the refrigerator for 3-4 days.