I love my cheesy au gratin potatoes but when we being strict about low carbs, potatoes are 'out' and other replacements are 'in'. Sometimes cauliflower can replace potatoes but other times, things like leeks, jicama or turnips fit the bill.
In this dish, turnips do just fine!
When I make these the traditional way, with potatoes, I also use 1 tablespoon of flour to thicken the sauce. If you can't do gluten you obviously need to leave this out, but those just doing 'low carb', you can either leave it out or leave it in. One tablespoon of all purpose flour is about 6 carbs and this is for the WHOLE casserole. Serving this casserole to 8 people is .75 of a carb added. Because of the other ingredients in this recipe it's going to run about 10-12 carbs per serving as it is depending on how big your servings are.
Note the bread crumb topping. This isn't 'needed' but adds so much to this dish if you are 'used to it' on regular potato au gratin. You have a lot of options - use what you have on hand. Pork rinds? Crush or process and use them! Gluten Free Panko crumbs if you can do some carbs but no gluten. If you have any low carb or keto bread on hand, run it through the food processor! Instant bread crumbs. That's what I used as I do not like most low carb almond flour based breads so when I try a new one and don't care for it or have half a loaf left and it's been a couple days... whirl it in a food processor, put it in a Ziploc in the freezer and pull it out any time you need a bread crumb topping! Great on chicken breasts, a mozzarella stick coating or... topping casseroles like these.
Half it if you wish.
Place the the thin sliced turnips in a greased oven-safe pan. Melt 1/4 c butter in a pan on the stove, add the onion and cook til soft and translucent. Add the flour if you are using it. Add salt and pepper. Stirring it all as it starts to bubble, add your cream/half and half. Whisk or stir until it just comes to a simmer/boil. After 1 minute, turn down the heat to low and stir in the cheese, adding by small additions to melt. Stir until smooth. Remove from heat. Pour over the turnips in the dish. Bake at 375. After about 40-45 minutes, add extra cheese or your optional bread crumb topping. Bake another 20 minutes til it's golden brown and thickened. Let it set for 10 minutes to 'set up' and continue to thicken before serving.
Turnips are so pretty. Like snowballs.
I thin sliced mine in my food processor but you can do it with a hand grater (usually I do) or with a knife - although it is hard to get a nice uniform 'thin' that way.
I wish I had 2 more turnips. I didn't. So mine was 'extra' creamy.
I used some keto bread crumbs I keep in the freezer from some of my 'trial' keto bread recipes.
You might also be interested in these low carb and/or keto crumbs available online or many other items; through Amazon if you can't find them locally.