This recipe from Smitten Kitchen has the same homey appeal (and high calories) as Mac n’Cheese, but with a whole lot more nutrition. When I cook chard and kale I usually discard the stems, but if you like them you can add them a few minutes before you throw in the leaves in. Note that chard cooks more quickly than kale, but since you’re making a gratin here it’s not that big a deal to cook them for the same amount of time.
Wash 1 bunch of chard and 1 bunch of kale. Remove the stems. Chop the greens roughly. Chop the stems finely if you plan to use them.
Trim 1 medium or 2 small leeks, then cut in half and rinse. Thinly slice in half rounds. Saute the leeks in 2 T. olive oil or butter in a heavy bottom pot until they are soft. Add the chopped chard and kale stems (if using) plus salt and pepper, and continue cooking for 8 minutes, until the vegetables are very soft but not yet browning.
Add the greens in handfuls, stirring to combine. Cook until they are completely wilted, then transfer into a colander and let them drain.
Make a bechamel sauce by combining 1 C. milk and 1 C. cream (or 2 C. half ‘n half) in a saucepan with 2 cloves garlic, minced. Simmer and keep warm. In a saucepan, melt 2 T. salted butter and add 1 T. flour. Whisk the roux for one minute, then add the milk/cream mixture while whisking constantly. Season with salt and pepper.
Thinly slice 2 lbs. of peeled or scrubbed sweet potatoes on an angle, 1/8″ thick. Grate 1 C. Gruyere Cheese.
In a 9 x 13 baking dish, make a thin layer of the greens. Then make a single layer of sweet potatoes. Sprinkle with salt, pepper, 1/3 C. of the cheese, and a pinch of fresh or dried thyme and parsley. Pour half the bechamel sauce over the sweet potatoes. Repeat with the rest of the greens, sweet potatoes and bechamel. Top with the remaining cheese.
Bake for 1 hour, uncovered, at 400 degrees. The gratin should be golden and bubbling. Let it sit for 10 minutes before serving.