Begin by preparing simple syrup. Add rhubarb, agave, strawberries, and salt to a small pan over medium heat. Pour 1 cup of water over everything and bring it to a boil. Reduce to a simmer and cook for about 15 minutes, until rhubarb pieces have softened and are very stringy. 

Carefully pour mixture into a fine mesh strainer, collecting the syrup in a jar underneath. You may need to press on the rhubarb a bit to ensure all of the liquid is squeezed out. Reserve fruit pulp for use in another recipe, like waffles or muffins. Set jarred syrup aside (you will have plenty left over).

To an ice-filled cocktail shaker, add orange juice, 2 oz rhubarb syrup, tequila, and orange liqueur. Shake for 20 seconds, until chilled. Strain into an ice-filled glass and garnish with fruit as desired.