Turnip, potato & mustard gratin - Testing

http://www.bbcgoodfood.com/recipes/166607/turnip-potato-and-mustard-gratin

1 - Trying this for the first time, I have not allowed enough baking time, so it's raw and in the refrigerator for tomorrow. Apart from the potatoes turning purple, keeping it for what turned out to be 4 days was fine, just not recommended. This is a very mild and kind of sweet dish. It was good with a salad and fresh bread, but is better as a side dish instead of a main. I will keep it at the Testing stage to bake it on the same day as I make it. OH! And also, the original instructed thick slices and I am changing it to thin. It may reduce the baking time as well.

2 cups water
2 cups milk 

Salt and pepper
1½ lb (720g/25oz) turnip or rutabaga, sliced thin
2 lbs (907g/32oz) potatoes, sliced thin
butter, for greasing
1 garlic clove, halved
1½ cups heavy cream
3 Tbsps. Dijon mustard 

1. Heat oven to 350°F. 
2. Fill a saucepan with the water, the milk, a good pinch of salt and a grinding of black pepper, then bring to the boil. Add the turnips and potatoes and cook for 2 mins. Strain, drain well and pat dry.
3. Butter a baking dish and rub the sides with garlic to just add a hint of flavour. 
4. Mix the cream and mustard in a large bowl, add the potatoes and turnip, mix until well coated. Seasoning well in between layers, fit all the potato and turnip in alternating layers into the baking dish, flattening the top level. 
5. Bake for 40 to 60 mins or until golden brown and the turnip and potato are cooked through – check with the point of a sharp knife.