Add rhubarb, strawberries, organic cane sugar, water and orange juice (optional) to a medium saucepan and bring to a simmer over medium heat.
Once simmering, reduce heat to low and continue cooking until softened and it looks like pie filling - about 5 minutes. While cooking, use a wooden spoon to stir and mash the fruit.
Remove from heat and let cool slightly, then add a fine mesh strainer over a mixing bowl and pour over the mixture. Use a spoon to make sweeping motions across the bottom of the mesh strainer to encourage the liquid to strain. Continue until you've extracted as much of the liquid "syrup" as possible.
Transfer syrup to a glass jar to the refrigerator to cool - at least a few hours. To speed cooling time, place in freezer for about 45 minutes to 1 hour.
Run a lime wedge around desired number of serving glasses and dip in salt or organic cane sugar (I like salt for a flavor contrast). Add a large ice cube (or several small ice cubes) to the glass, and set aside. Set in the freezer for an extra chilly margarita!
Next, to make one margarita, add 75 ml strawberry rhubarb syrup (amount as original recipe is written // adjust if altering batch size) to a cocktail shaker, along with lime juice, tequila, agave nectar, and a handful of ice.
Shake vigorously to combine. Pour out a small amount to sample, then adjust flavors as needed, adding more agave nectar for sweetness, lime juice for acidity, or strawberry rhubarb syrup for fruitiness.
Pour over serving glass and enjoy! The syrup makes enough for about 3-4 margaritas, and will keep in the refrigerator covered up to 1 week (sometimes longer).
*I used organic cane sugar, but you could try coconut sugar or even maple syrup. However, know that these will likely add a coconut or maple flavor. Stevia would be a good natural alternative (add to taste). *Nutrition information is a rough estimate calculated with lesser amount of agave nectar and without orange juice.