Creamy Colcannon Potato Gratin

What to cook on St Patrick's day?  I love dauphinoise potatoes and I love colcannon so why not combine an idea of the two?  
It's a triumph of luxurious loveliness...not particularly slimming but never mind, we can eat salad tomorrow!  This is wonderful as a side dish or particularly comforting just on its own. 
   

Serves 8 as a side dish or 4 in a more central role

Ingredients
About a kilogram of Potatoes, peeled and sliced very thinly 
1 Large Onion, thinly sliced
Half a Large Green Cabbage, thinly sliced (Sweetheart is good but any can be used)
A splash of White Wine (couple of tablespoons)
1 Garlic Clove, minced
Knob of Butter
Drizzle of Olive Oil
300ml Double Cream (or a mixture of cream and milk)
200g Mature Cheddar, grated (Gruyere is also good)
Salt and Pepper

• Preheat the oven to 175C (fan).
• Put the potato slices in a large pan of water and just bring to the boil, turn off the heat immediately and drain into a colander, leave to dry.
• Put a large frying pan onto the heat and add a drizzle of olive oil and the knob of butter.  When melted, tip in the sliced onion and sizzle on a low heat for about 10 minutes until soft then add the cabbage and the minced garlic.  Splash in the white wine and allow to bubble away for a few minutes then pour in 250ml of the double cream (or the cream/milk mixture).  Bring it back up to the boil and simmer for about 5 minutes, season generously with salt and pepper.
• In the bottom of a large oven proof dish spread out a spoonful of the cabbage mixture, sprinkle with a handful of cheese then put on a thin layer of potatoes.  Continue to layer the potato slices, cheese and creamy cabbage finishing with a layer of potatoes on the top.  Pour the remaining 50ml of cream over the top, sprinkle with cheese then season with salt and freshly ground black pepper.
• Cook in the preheated oven for approximately 40 minutes or until golden brown.  It's also good to check the potato is soft by inserting a knife into the middle of the gratin, if the potatoes seem at all firm, pop it back in the oven for 10 minutes (you can cover it with foil if you're worried about the top browning too much).