Prepare the cabbage base: Preheat your oven to 400°F (200°C). Remove the outer leaves of the cabbage and slice the cabbage into thick, round steaks about 1 inch thick. Brush both sides of each cabbage slice with olive oil and season with salt and pepper.

Roast the cabbage: Place the cabbage slices on a lined baking sheet and roast them in the oven for 25-30 minutes, flipping halfway through, until the edges are golden brown and the cabbage is tender.

Add toppings: Once the cabbage slices are roasted, remove them from the oven and spread a thin layer of whole grain mustard on each slice. Top with slices of corned beef and sprinkle the shredded Dubliner cheese evenly over the top.

Bake until bubbly: Return the cabbage pizzas to the oven and bake for an additional 10-12 minutes, or until the cheese is melted and bubbly.

Garnish and serve: Remove from the oven and garnish with chopped scallions. Serve warm and enjoy this flavorful, low-carb alternative to traditional pizza.