Preheat the oven to 375 degrees.
Place the chopped butternut squash in a medium roasting pan. Pour the extra virgin olive oil over the squash and sprinkle with ¼ teaspoon chili powder. Roast for about 25 minutes, or until tender but not mushy. Stir the squash every 10 minutes while it’s roasting.
Meanwhile, drain and rinse the black beans. Place them in a medium skillet on the stovetop and sprinkle with the additional 1/4 teaspoon of chili powder (or to taste). Warm them gently over medium low heat, stirring occasionally. Remove from heat and set aside to cool.
Prepare the radishes, scallions, tomatoes, and avocado. Place them in a large bowl. Dress lightly with citrus vinaigrette (up to 2 tablespoons, to taste).
When the squash is tender, remove it from the oven and let it cool for about 10 minutes. Transfer the squash to a separate medium bowl, and add the black beans. Add ½ teaspoon of salt (as needed). Taste the mixture, and add additional chili powder or salt if desired. Let the mixture cool to room temperature.
Add the radish mixture to the squash and beans. Stir gently to mix the ingredients.
Place a small bed of arugula leaves on each of six plates. Divide the taco salad evenly among the plates and serve.