Peel lemons, taking only the zest and avoiding the white pith.

Place rosemary sprig in a 1-gallon glass or ceramic container.

Add lemon zest to the container.

Pour 750 ml of vodka over the rosemary and zest.

Seal the container and let it sit in a cool, dark place for 40 days.

In a saucepan, bring 5 cups of water to a boil.

Add sugar and cook, stirring, until sugar has dissolved, creating a simple syrup.

Let the sugar syrup cool to room temperature (about 1 hour).

Pour the cooled syrup and the remaining 750 ml of vodka over the lemon-vodka mixture.

Stir to combine and seal the container.

Let it sit in a cool, dark place for another 40 days to allow the flavors to meld.

Pour the limoncello through cheesecloth into a large spouted pitcher to strain.

Divide the limoncello among gift bottles for storage and serving.