Bring a large pot of water to a boil over high heat. Add 2 tablespoons salt. When the water returns to a boil, stir in the orzo and cook until al dente, about 6 to 7 minutes. Drain well.
Return the cooked orzo to the pot, stir in the butter, and place over high heat. Add the mustard greens and lemon zest and cook, stirring until the greens wilt, about 3 to 5 minutes. Season to taste with salt and pepper. Serve with the freshly grated cheese on top.