Preheat oven to 450ºF.
Arrange the waffle fries on a baking sheet and cook according to package directions, adding about 10 minutes onto the baking time to make them extra crispy.
When the fries have about 10 minutes left, place a medium skillet over medium-high heat with 2 turns of the pan of EVOO, about 2 tablespoons. Add the corned beef to the skillet and sauté until light golden brown and heated through, 2-3 minutes.
Add the onions and cabbage to the pan, and cook until the onions have softened and cabbage has wilted, 4-5 minutes. Season with salt and pepper and reserve warm.
While the veggies are cooking, place a medium pot over medium-high heat and melt the butter. Add the flour to the pot and cook for about a minute.