A cocktail shaker would be great, if not, just stir it all up well in a glass.
Instructions
Rhubarb Syrup
Cut up the rhubarb stalks into about 5cm/2" pieces.
Grate, chop or pound the ginger, don't peel it, just scrub it clean.
Place everything into a small saucepan and bring to boil. Then simmer for 10 minutes, by which time, the rhubarb would be very soft and pulpy.
Strain the syrup into a clean jug, pushing down the rhubarb to extract as much of the flavour as possible. For a clearer margarita, don't squash down, just strain the liquid through.
Leave to cool. You can place it in the fridge, if you like.
Rhubarb Juice
If using a juicer, just juice the stalk.
If using a chopper, cut up the stalk into 3 pieces and blitz in the chopper.
Strain, squeezing out all the liquid.
Salt and Ginger Rim
Mix the ginger and salt together and place in a saucer.
Serving
Run a lime slice all around the rim of a glass.
Dip the glass rim into the ginger flavoured salt, coating well.
Method One:
Place everything in a shaker, and pour into the glass, minus the ice cubes.
Serve with a slice of lime on the rim.
Method Two:
Pour the rhubarb syrup into the glass.
Mix everything else in the shaker and pour over the syrup very slowly. You could do it over a spoon if you like, but it shouldn't be necessary, given the density of the syrup. This second method will create the layers as you see in the image. Serve with a little straw or shaker, with a lime on the rim.
Notes
Total time does not take into account the cooling down of the rhubarb syrup.