Bake in the preheated oven for 20–25 minutes, or until the edges are lightly golden, the top is just starting to brown in spots, and the butter has melted into shiny puddles across the surface.
Remove the casserole from the oven and let it sit for about 5 minutes before serving. Bring it to the table in the baking dish so everyone can see the creamy potatoes, tender cabbage, and melted butter swirled together, just the way my grandmother used to serve it.
Variations & Tips
If you’d like a bit more color and sweetness, you can fold in a small handful of very finely sliced green onions along with the cabbage, though my grandmother usually kept it to just the four basics. For a slightly richer casserole, warm an extra tablespoon or two of butter with the milk before mashing, or drizzle a little more melted butter on top right as it comes out of the oven. If you prefer a looser, creamier texture, add a splash more warm milk when mashing the potatoes until they reach your favorite consistency.
For a make-ahead option, assemble the casserole earlier in the day, cover, and refrigerate; when ready to bake, let it sit at room temperature for about 20–30 minutes, then bake until hot and bubbly, adding a few extra minutes if it’s starting out cold. Leftovers can be turned into potato cakes: form chilled colcannon into small patties and pan-fry them in a little butter until browned on both sides. And if you’re cooking for a smaller household, the recipe halves easily—use a smaller casserole dish so you still get that nice, cozy, golden-topped look.
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