Add the peeled and diced potato to a pot of cold water, cover, place over a high heat and bring to the boil. Reduce heat and simmer until the potato is tender when pierced with a fork.
Place a metal steamer into another pot, add a little water and bring to the boil. Place the cabbage into the steamer and steam cook until it is tender.
When the potatoes are cooked, remove from the heat, drain into a colander, then add back into the pot with the butter and milk. Using a potato masher, mash the potatoes until smooth and creamy. You may want to add a little bit more or less milk and butter, it is up to you!
Add in the spring onion, steamed cabbage, sea salt and black pepper and stir through with a spoon until evenly combined.
Serve the colcannon straight away with a little extra butter and sea salt if you want.