Whisk buttermilk, lemon juice, zest, and honey in a large bowl. Pour into the bowl of an ice cream maker and follow manufacturer’s instructions to freeze. When sherbet is frozen, pour into a large bowl, cover and place back in the freezer.
For Stewed Rhubarb:
Place rhubarb, honey and lemon juice in a medium saucepan over medium heat. Cook for about 5 minutes, stirring frequently, until rhubarb softens, but doesn’t lose its shape. Remove from heat. Serve stewed rhubarb over sherbet.