Preheat the oven to 375˚. Combine the lime zest, fajita seasoning and 1/2 teaspoon salt in a small bowl; rub all over the chicken. Heat a large nonstick ovenproof skillet over medium-high heat. Add the corn and cook, stirring, until warmed through, 30 seconds; remove to a bowl. Add the vegetable oil to the skillet. Add the chicken and cook until browned, 3 to 4 minutes per side. Transfer the skillet to the oven and cook until the chicken is just cooked through, about 12 minutes. Transfer to a cutting board to rest 5 minutes.
Meanwhile, combine 1 cup spinach, the halved avocado, buttermilk, lime juice, 1/4 teaspoon salt and a few grinds of pepper in a blender. Puree until smooth. Blend in 2 to 4 tablespoons water until thin enough to drizzle.
Divide the remaining spinach among shallow bowls. Arrange the corn, sliced avocado, tomatoes, cabbage and tortilla strips on top. Slice the chicken against the grain and add to the bowls. Drizzle with some of the dressing and serve the rest on the side.