Add cucumber slices, lime juice, triple sec, tequila, and ice to a blender.
Blended until margaritas are smooth.
Run a lime slice around the rim of each rocks or margarita glass to moisten it. Dip into Tajín Clásico Seasoning Rimmer or margarita salt to coat the rim of each glass.
Pour margaritas into prepared glasses, being careful not to knock off the rim.
Top each margarita with a tablespoon of Grand Marnier, and garnish with lime slice.