With its trademark tart flavor, rhubarb makes a perfect palate-cleansing sorbet between courses, especially if you're serving rich meats.
Place the rhubarb in a medium pan and cover with 3/4 cup water. Bring the liquid to a boil over high heat, then reduce the heat to medium-low and simmer until the rhubarb is tender, stirring occasionally, about 10 minutes.
Remove the pan from the heat and let the rhubarb cool for 5 to 10 minutes. Transfer the mixture to a food processor and purée until smooth. Pass the purée through a fine mesh sieve, discarding solids. Refrigerate, covered, until completely chilled.
Meanwhile, make a simple sugar syrup by placing the sugar and the vanilla bean in a medium-size bowl. Bring about 2 cups of water to a boil, and then add 1 1/4 cups of the boiling water to the bowl. Stir until the sugar dissolves. Pour the syrup with the vanilla bean into a clean container with a lid, and let sit at room temperature until cool. Then, refrigerate the syrup until it's completely chilled.
Before making the sorbet, remove the vanilla bean. Scrape out the tiny seeds with a small knife and add the seeds to the syrup. Wash the vanilla pod and save it for another use. Add the syrup to the chilled rhubarb purée. Stir in the lemon juice.
Transfer the mixture to an ice cream maker and churn according to manufacturer's instructions or until the sorbet is firm.
To store, transfer the sorbet to a clean container, press a piece of plastic wrap against the surface of the sorbet, cover with a lid, and freeze. If the sorbet is too hard to serve, let it soften in the refrigerator for about 20 minutes before serving.