Make the puree: In a small blender or food processor, blend ½ cup fresh watermelon with lime juice and simple syrup until smooth. Strain if desired for a smoother texture.
Rim the glass: Run a lime wedge around the rim of your glass and dip it into a plate of Tajín to coat.
Shake it up: In a cocktail shaker filled with ice, add 1 oz tequila, 1 oz watermelon puree, and a thin slice of deseeded jalapeño. Shake for 30–60 seconds until chilled.
Build the drink: Fill your prepared glass with ice. Pour the shaken mixture in until about halfway full.
Top it off: Add 1 oz of Topo Chico and a splash of Grand Marnier on top as a floater.
Garnish: Add a lime wedge and an extra jalapeño slice for a spicy touch. Serve immediately and enjoy cold.