In a food processor fitted with an S-blade or a blender, blend the garlic, cilantro, jalapeno, avocado, oil, and salt until smooth. Transfer to a jar and refrigerate until ready to use. Can be made up to one day in advance.
Combine the cabbage, green onions, and cilantro. In a small bowl, whisk the oil, lime juice, sugar, and vinegar. Pour over the cabbage and toss to coat. Set aside. Slaw should be made up to one day in advance. The longer it sits, the softer the cabbage becomes.
Place fish in a wide, shallow bowl and season with chile powder, lime zest, salt, and pepper. Pour the lime juice and olive oil over the fish and rub well to coat. Preheat a grill pan or skillet on medium heat for 2-3 minutes. Add the fish and grill until flaky and cooked through, about 3-4 minutes per side. Transfer to a plate before assembling the tacos. Alternately, you can bake the fish in a 350F oven for 10-12 minutes.
To serve, warm up the corn tortillas in the oven for 1-2 minutes. Layer the fish, a handful of cabbage slaw, and a drizzle of verde sauce and serve immediately. Enjoy!