Swiss Chard & Sweet Potato Gratin

I'm in love. This is one of the best gratins I have EVER had. It's rich, creamy, a tad sweet with wonderful flavors of chard and Gruyere cheese. I had this for dinner with a side of steamed broccoli. Nothing else needed. The recipe is originally from Smitten Kitchen, but needed to be adapted. First I needed to make it gluten free, and second I didn't want to make a dish that served 12 people! You will be pleasantly surprised. It is fantastic!

Serves 4-5
1/4 stick butter
1/2 small white onion, finely chopped
2 bunches swiss chard, cleaned, leaves only and cut into 1 inch pieces
pinch of nutmeg
1 garlic clove, minced
1 cup of heavy cream or whole milk
1 tbs gluten free flour
1 lb red-skinned sweet potatoes, peeled and cut into 1/8 inch thick rounds
1 tsp fresh flat leaf Italian parsley, minced 
1 tsp fresh thyme, minced
sea salt and fresh ground pepper
1 cup Gruyere cheese, coarsely grated

Prep the Chard: Heat 1 tbs butter over medium heat in wide 6-qt pot, add onions and cook until softened. Add greens and nutmeg, stir until all leaves are wilted. Season with salt and pepper. Transfer to a colander and drain well. Press out excess moisture with the back of a large spoon. 
Make Sauce: combine milk and garlic in a small sauce pan. Bring to a simmer and keep warm. In a larger sauce pan, heat 1 tbs butter and add GF flour, whisking constantly (do not let burn). Slowly add in milk/garlic and continuing whisking , bringing to a simmer. You have just made a gluten free roux! Season with salt and pepper. Remove from heat and set aside. 
Assemble Gratin: Preheat oven to 400°F. Butter a 8x8 baking dish (I used an odd 6 x12 oval shaped dish). Spread 1/2 the sweet potatoes in the dish. Sprinkle with salt and pepper and 1/4 of the herbs and 1/4 cup of the cheese. Now top with 1/2 of the greens, 1/4 of the herbs and 1/4 cup of cheese. Now poor 1/2 of the milk mixture over the 2 layers. Season with salt and pepper. Add the remaining sweet potatoes, remaining herbs, 1/4 cup of cheese, the rest of the greens and again season with salt and pepper. Pour the remaining 1/2 of the milk mixture over the greens (pressing the vegetables slightly, to make sure they are submerged and moist). Sprinkle with the last 1/4 cup of cheese. 
Bake for 45 minutes until golden and bubbly. Let stand for 10 minutes before serving.