Place the sliced jalapeño in the bottom of a glass container (like a mason jar or pyrex measuring cup) and set aside. Add the water and agave nectar to a small pot and heat over medium heat. Stir well and heat the mixture until it comes to a simmer. Pour the simple syrup over the jalapeño. Cool to room temperature, cover and refrigerate at least 6 hours and up to 48 hours to infuse the jalapeño into the simple syrup.

After infusing, pour the simple syrup through a sieve into a container to strain the seeds and jalapeño. Store in the refrigerator until ready to use, up to two weeks.

Rim two glasses with margarita salt by rubbing a lime wedge along the rim of the glasses. Dip the glasses in the salt. Place a slice of jalapeño in the bottom of each glass and muddle or press squish a bit with a spoon. Fill glasses with ice.

Add the tequila, grand marina, lime juice and jalapeño simple syrup to a cocktail shaker. Shake well and pour over the two prepared glasses. Garnish with an additional slice of jalapeño and a slice of lime.