Trim the rhubarb stalks to fit two widemouth pint jars. Slice the stalks lengthwise if they're thick. 

Divide the mustard seeds, peppercorns, cloves and star anise between two sterilized widemouth jars (see note). Pack the rhubarb stalks snugly into the jars. Set aside.

Combine the apple cider vinegar, water, sugar, and salt in a small saucepan and bring to a boil over high heat. Cook, stirring, until the sugar and salt have dissolved. Pour the liquid into the jars over the rhubarb, leaving 1/2 and inch of space. Tap the jars lightly with a wooden spoon to dislodge any bubbles. Wipe the jar rims with a damp, clean cloth and apply lids and rings.

Process the jars submerged in boiling water for 10 minutes, starting the 10 minutes when the water returns to a boil after adding the jars. Remove jars and set to cool on a kitchen towel. When cool enough to handle, remove rings and test the seals. Store sealed jars in a cool dark place for up to a year, and any unsealed jars in the refrigerator, consuming with 2 weeks. Allow the pickles cure for at least 48 hours before eating for maximum flavor!