"This flavorful, hearty-textured gratin made with Brussels sprouts and kale is craveable. The celery root takes on a creamy texture when cooked and the mustard in the topping adds a mild bite. Skip the ham to make this side vegetarian...."
INGREDIENTS
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2 tablespoons unsalted butter, melted, plus more for greasing
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3 tablespoons whole-grain mustard
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1 1/2 cups coarse whole-wheat breadcrumbs or plain breadcrumbs
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1/4 cup fresh parsley leaves, roughly chopped
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2 tablespoons sliced fresh chives
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Kosher salt and freshly ground pepper
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3/4 cup heavy cream
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3/4 cup chicken or vegetable stock
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1 medium celery root (about 12 ounces), peeled, quartered and thinly sliced
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2 tablespoons olive oil, plus more for drizzling
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One 7-ounce ham steak, cut into 1/2-inch dice, optional
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2 medium shallots, sliced
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1 pound Brussels sprouts, trimmed and quartered
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1 bunch kale (about 8 ounces), stemmed and sliced
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1 tablespoon apple cider vinegar