On a hot grill, or cast iron skillet, char halved grapefruits, fruit-side down, along with ginger and serrano.
Once charred, place ginger and serrano in a blender, and add agave, W Sauce®, and 1 cup of purified water. Blend, and strain into a small pitcher with a fine-mesh strainer or cheesecloth. Set aside.
Juice grapefruit halves and strain into another small pitcher. Set aside.
In a cocktail shaker, add ice, 1 oz ginger serrano W Sauce simple syrup, 1 oz grapefruit juice, ½ oz orange liqueur, 2 oz tequila or mezcal, and shake!
Wet a glass rim with a grapefruit wedge, and in a shallow bowl, sprinkle with margarita salt. Dip the rim of the grapefruit-kissed glass in the salt. Fill the glass with large cocktail ice cubes. Pour the cocktail with a strainer over ice in the glass. Garnish and serve.