How to Make It
Cut away the peel and pith of the grapefruit. Working over a small bowl, cut along both sides of each grapefruit segment, releasing the segments into the bowl. Squeeze the membrane to release 1 tablespoon of the juice. Add the avocado, 1 tablespoon of the oil, and ¼ teaspoon each salt and pepper and toss to combine.
Heat 1 tablespoon of the remaining oil in a medium skillet over medium heat. Add the onion and ¼ teaspoon each salt and pepper and cook, stirring occasionally, until soft, 4 to 5 minutes. Add the chicken and chilies and cook until warmed through, 2 to 3 minutes more.
Meanwhile, heat the remaining oil in a small saucepan over medium heat. Add the beans and scallion whites and cook until warmed through, 3 to 4 minutes. Stir in the vinegar. Divide the chicken mixture among the taco shells and serve with the salad, beans, and salsa. Sprinkle with the scallion greens.