In the bottom of a rocks glass, muddle the rum, strawberries, lime juice, mint leaves, and agave nectar together. Make sure the agave is really well-incorporated.
Top the glass off with ice, then with sparkling water. Garnish with a strawberry, and serve.
Notes
Less-Sweet Variation:If you want to shoot for a lower-sugar drink, muddle together everything in step 1 except the agave. Then taste, and add the agave, if needed, ⅛ teaspoon at a time before moving on to step 2. Remember that the sparkling water will dilute the rum a bit, so you're not trying to totally mask the boozy taste in this step.