If you're going to use the orange zest, wash the orange well and use a vegetable peeler to remove a long strip of the orange part of the peel.
Put the rhubarb, strawberries, honey, salt, water, and orange zest and/or cinnamon stick, if you're using it, in the pot.
Put the pot on the stove and turn the heat to medium-high. Bring the mixture to a boil (you'll see lots of bubbles around the inside edges of the pot) and stir to dissolve the honey.
Turn the heat down to low and simmer ("Simmer" means to cook at a very gentle boil), stirring occasionally, until the rhubarb and berries become totally mushy, about 8 minutes. Set the mixture aside and let it cool to room temperature. Throw away the orange peel and/or cinnamon stick.
Set the strainer over the bowl and pour the rhubarb mixture into the strainer. Stir the rhubarb with a wooden spoon or rubber spatula to help it go through the holes. (You should end up with 3/4 cup of rhubarb puree; the leftover pulp in the strainer is delicious mixed with some yogurt, or you can compost it.)
Divide the puree evenly among the 6 glasses. Top each with 1 cup sparkling water and stir gently. Add ice cubes and serve.