Swiss Chard and Sweet Potato Gratin

I was so excited to make this dish since we have a not insignificant quantity of chard to finish off from our garden. But making this dish by smittenkitchen made me realize I have absolutely no concept of how much chard it takes to reach 3 lbs worth of greens! At the final count, I ended up with about 2.5 lbs of greens (according to our cheap Ikea scale...) which included chard, spinach, and kale - in addition to some small broccoli florets (why not?).

I ended up cooking the greens and onion about 5 days in advance, whoops! talk about a delay in cooking, but they were no worse for wear when I finally assembled the casserole. Note: owning a mandolin makes slicing 2 lbs worth of 1/8" thick slices of sweet potato fairly easy. I also assembled the casserole in the morning and took it out of the fridge an hour before I planned to start baking it. The dish was in the oven when I turned it on to preheat. These strategies will prevent breaking a glass baking dish. If you have a metal baking pan, no worries.

The bechemel sauce for this dish smells so incredible I was in danger of eating it all before putting any in the casserole. Fortunately I held off, believing that the bechemel in combination with the sweet potatoes, chard, grated gruyere and herbs would be worth the wait. Oh the smell of this casserole filling the house - mouthwatering, stretching patience to the limits! The finished casserole more than surpassed the anticipation and expectation in every deliciously rich bite. I did find the sweet potato flavor very sweet - partly because the recipe has no instructions on how much salt is needed. If I made the dish again, I would try half potato/half sweet potato to tone the sweetness down.

Sweet Potato-Chard Casserole
1/4 cup butter
1 medium onion, finely chopped
3lbs chard (or a combination of kale/chard/spinach), leaves and stems separated, cut into 1" pieces **if you use kale, don't use the stems at all!**
1/4-1/2 tsp of freshly grated nutmeg
1 c. whipping cream 
1 c. evaporated milk
4-6 garlic cloves, minced
2 Tbs. butter
2 Tbs. flour
2 lbs sweet potatoes (orange ones), peeled and cut into 1/8" thick slices
1/4 c. minced fresh Italian parsley
1 Tbs + 1 tsp thyme
Fine sea salt
Freshly ground black pepper
1 1/4 c. (about 6 ounces) coarsely grated Gruyére cheese

1. Melt 1/4 c. butter in a large skillet or heavy pot. Add onion, saute over medium low heat until softened. Add kale leaves (if using) and saute for 10-15 minutes until softened. Add chard stems, nutmeg, salt and pepper, keep sauteing, until veggies are tender. Add chard leaves (and spinach, if using) by large handfuls, stir, cover, and let greens wilt (occasionally stirring as needed - you can also add a bit of water if things are sticking to the bottom).
2. Once the greens are wilted, transfer to a colander to drain and press out liquid using a large spoon. (I saved the liquid to add to stock for soups - it's tasty!). Set the chard aside to cool. You can add more salt and pepper to taste as needed.
3. If you haven't already, slice up the sweet potatoes. Submerging them in water will keep them from browning, but they need to be dried before putting in the casserole dish. (Press them with paper towels to dry them). Also, grate the cheese and set aside. Mince the parsley and set aside.
4. Bring cream, evap milk, and garlic to a simmer in small saucepan. Keep covered and set aside.
5. Melt 2 Tbs butter in a medium heavy saucepan over moderate heat and stir in flour. Cook roux, whisking, until mixture gently bubbles/boils. Keep whisking - roux should start to smell cooked and brown ever so lightly (~1-3minutes). Slowly whisk in cream mixture and bring to a boil, continuously whisking, one minute. Add salt and pepper.
6. If you're going to bake the gratin straight away, preheat oven to 400 degrees F. If not, start assembling the gratin: Butter a deep 9×13 baking dish (my pyrex one worked fine). Spread half of the sweet potatoes over the bottom of the dish. Sprinkle with salt, pepper, some parsley/thyme, and a 1/4 c. of Gruyere. Layer half of the chard mixture next, then salt, pepper, parsley/thyme, and 1/4 c. of Gruyere. Pour half of bechamel sauce over the first two layers. Repeat with rest of sweet potatoes, s/p/herbs/cheese, chard, s/p/herbs, rest of the bechemel sauce, then last 1/4 c. of cheese. Try and press everything down into the pan so that the veggies are submerged in sauce. Don't worry if it doesn't look like enough sauce (mine didn't) and it bubbled up just fine/staying quite moist
7. Bake gratin for about 1 hour until golden and bubbly, and most of the liquid is absorbed. Let stand 10 minutes before serving.(I baked mine for about 1.5 hours since it had been in the fridge, the first hour with foil on top, and the last 30 minutes uncovered).