Preheat the oven to 450F. Cover the potatoes with water in a pot that will accommodate them, bring to a boil and cook until they’re just fork-tender. Drain. (You can do this up to a few days in advance, and keep them in the fridge.) Lay them out on a baking sheet and flatten each with a fork. Drizzle with oil, sprinkle with salt and roast for 15-20 minutes, or until crispy and golden. If you like, drizzle the cabbage wedges or slices with oil and roast them alongside on the baking sheet. Otherwise get a heavy skillet nice and hot, add a generous drizzle of oil and cook for 5-8 minutes per side, until deep golden and starting to blacken on both sides, and tender-crisp in the middle. (If it doesn’t seem to be cooking through the middle, add 1/4-1/2 cup water or stock and cover the pan for a few minutes to allow it to steam a bit. I avoid this because the cabbage can be a bit more wilted, but some people like that too!)
To make the sauce, combine the cilantro, mayonnaise, sour cream, jalapeño, garlic, lime juice and salt in a food processor or blender and pulse until smooth. Drizzle it over the cabbage and potatoes, and if you like, top with a few curls (make them with your vegetable peeler) of aged Irish cheddar or other cheese. Serves 6ish.