1. Salt the rim of the glass by rubbing a grapefruit wedge around the edge and dipping it onto a small plate of salt.
2. Shake the tequila, grapefruit juice, rhubarb syrup, and lime juice in a cocktail shaker with ice.
3. Fill the glass with ice and strain drink over ice. Garnish with thinly sliced rhubarb.

For the rhubarb syrup:

1. Combine 4 cups chopped rhubarb, 1 cup granulated sugar, and 1 cup water in a heavy-bottomed saucepan and bring to a boil. Lower the heat to a simmer and cook gently, stirring occasionally, until the fruit is soft and the liquid has thickened slightly, about 20 minutes.
2. Set a fine-mesh strainer (or a coarse strainer lined with cheesecloth) over a large bowl. Pour the rhubarb through the strainer until most of the liquid is in the bowl. Press the solids a little with the back of a spoon to extract more syrup.
3. Carefully pour the syrup into a clean bottle. Cover or cork the bottle and refrigerate for up to two weeks.