Preheat the oven to 400º F. Line a baking sheet with parchment. Transfer the boiled potatoes to the lined baking sheet. Use a heavy drinking glass to smash each potato flat. Drizzle with olive oil and sprinkle with kosher salt and pepper. 

Bake in the oven until golden brown and the edges begin to crisp, about 15-20 minutes. Remove from the oven and set aside.

Meanwhile, bring a large pot of water to a boil. Add the cabbage leaves and cook until softened, about 2 minutes. Rinse under cold water, then drain and pat dry and set aside.

Mix the parsley and dill with the mayonnaise and set aside. 

Spread a thin layer of mayonnaise down the center of each cabbage leaf, vertically.

Place 1-2 smashed potatoes down the center of each cabbage leaf, on top of the herbed mayo. Top the potatoes with a slice of the corned beef, crinkle the meat as you lay it on the potatoes. Dollop the whole grain mustard on top of the corned beef.

Roll each filled cabbage leaf up, then fold the sides of the cabbage leaf under to close the sides.