Preheat oven to 425°F. Line a 15×10-inch baking pan with foil. Cut squash into ½-inch-thick slices, then cut slices into thirds. Place squash and onion in a bowl. Add 2 Tbsp. of the lime juice, the chili powder, and salt and black pepper; toss to coat. Spread in prepared pan. Roast about 25 minutes or until squash is tender and golden, stirring once.
Meanwhile, for salsa, in a bowl combine beans, corn, tomato, scallions, jalapeño, and the remaining 1 Tbsp. lime juice.
Fill tortillas with roasted squash mixture, salsa, sliced avocado, and fresh cilantro. Serve with lime wedges.
