In this elegant gratin, nutty farro and earthy mushrooms are baked with herbs and mascarpone into a creamy casserole topped with Gruyère. It’s a rich, meatless meal that needs nothing more than a crisp green salad to round it out, though it’s also a satisfying side dish alongside roast chicken or fish. You can prepare the entire gratin earlier in the day (leave it at room temperature), then bake it just before serving — simply add a little extra time to ensure that it emerges from the oven steaming hot through and through. Note that if you’re using tangy crème fraîche instead of mascarpone, you may not need the lemon. Taste before squeezing.