Place 1 teaspoon Diamond Crystal kosher salt and 1 teaspoon Tajín seasoning on a small flat plate and stir to combine. Spread into an even layer. Cut 3 half-moon slices from 1 small orange and reserve for garnish (save the remaining orange for another use). Run 1 orange slice over the rim of 2 old-fashioned or margarita glasses to moisten. Invert each glass into the Tajín mixture, turning it as needed, to coat the rim. Fill the glasses with ice.
Juice 2 medium limes until you have 2 ounces and place in a cocktail shaker. Add 4 ounces blanco tequila, 2 ounces Aperol, and 1 ounce agave syrup. Fill the shaker halfway with ice, seal, and shake until the outside of the shaker is very frosty, about 20 seconds. Pour through a fine-mesh strainer into the glasses. Garnish each glass with an orange slice.
