In a saucepan, melt butter over medium heat. Add rice and cook, stirring constantly, for 2 minutes. Stir in salt. Add chicken stock, lemon juice and chopped rosemary. Bring to a boil, stirring frequently. Cover with a lid, reduce the heat to low, and simmer until the liquid is absorbed and rice is tender, 15 to 20 minutes. Remove from the heat and let sit, covered, for 5 minutes. Fluff with a fork and stir in lemon zest. Serve immediately, garnished with fresh rosemary.