In a saucepan, combine all the ingredients and bring to a boil. Once boiling, reduce the heat to a simmer and simmer for 15 minutes. Let cool completely before straining.
Strain juice using a mesh strainer and store in an air tight container. Will last in fridge for 2 weeks.
Crush ice (if desired) and fill up your cocktail glass.
In a cocktail mixer, muddle the tequila and the mint leaves together just to release a little bit of the mint oil into the tequila.
Add Cointreau, lime juice and strawbarb simple syrup to cocktail shaker. Stir to combine.
Strain out into glass with crushed ice. Garnish with a lime wheel and a sprig of mint.
I don't care for a particularly sweet margarita but you can add as much simple as you want. If you don't find it sweet enough after you've poured it over ice, just pour on a little syrup as you go. I doubt the simple syrup will last 2 weeks in the fridge, it's so good you'll find yourself making it every night.