Combine the tequila, lime juice, agave, oil, Cointreau, ground cumin, and cilantro leaves in a food processor or blender and process until fully combined.
In a mixing bowl, combine the coleslaw mix, radishes, and red onion. Add just enough of the marinade to moisten the coleslaw, about 2-3 tablespoons. Season with ½ teaspoon salt and ¼ teaspoon pepper and let sit at room temperature while the fish marinates.
Season the fish with salt and pepper. Pour the remaining marinade over the fish in a food storage bag and let sit at room temperature for 15-20 minutes.
Heat a skillet over medium-high heat, brush with a little oil, add the fish, and cook for 4-5 minutes or until opaque, flipping once.
Serve on tortillas with the slaw and any additional garnishes you desire.