Whisk vinegar, sugar and salt in 1 cup water in a small bowl until sugar and salt dissolve. Place onion in a jar; pour vinegar mixture over onions. Let sit at room temperature for 1 hour. Can be made ahead and refrigerated up to 2 weeks.
AVOCADO CREAM SAUCE
Mash avocado. Mix avocado, yogurt, garlic, lemon juice, and pepper together until smooth. Refrigerate until ready to squeeze on tacos
TACOS
Layer black beans and butternut squash onto tortilla. Heat under broiler until hot.