Preheat the oven to 450 degrees F. Line a large rimmed baking sheet with parchment paper and set aside.
Hold the root-end of the cabbage. Slice the cabbage into six 3/4 inch rounds. Lay the cabbage rounds on the baking sheet and brush with olive oil. Then sprinkle generously with salt and pepper.
Roast the cabbage steaks in the oven for 15-20 minutes, until crispy around the edges.
Meanwhile, shred the Dubliner cheese. Chop the scallions. And slice the deli corned beef into 1/2 inch ribbons.
Brush 1 teaspoon whole grain mustard over the top of each roasted cabbage steak. Then top with corned beef and shredded cheese. Place the cabbage pizzas back in the oven for 5 minutes to heat the corned beef and melt the cheese.
Sprinkle with chopped scallions and serve warm.
Notes
Can't find corned beef or Dubliner cheese? You can swap the corned beef for pastrami, and the Dubliner for any Irish cheese, or even sharp cheddar.