Making homemade flavored vodka takes only a little effort but produces impressive results. The pear-spiked liquor can be stored at room temperature for up to two months, but needs at least two weeks to infuse the fruit's flavor, so plan ahead.
Place enough pears in a 56-ounce glass jar to fill to the top. Add vodka. Seal jar, and let stand at room temperature for at least 2 weeks. Strain vodka, and discard pears.
Heat sugar and water in a medium saucepan over medium-high heat, stirring, until sugar dissolves. Add rosemary. Remove from heat, and let stand for 30 minutes. Discard rosemary. Let cool completely.
Combine vodka, rosemary simple syrup, pear nectar, and sparkling water in a large pitcher. Divide among 8 ice-filled glasses, and garnish each with a rosemary sprig. Serve immediately.
Rosemary simple syrup can be refrigerated for up to 1 month.