Cut one delicata squash in half lengthwise. Scrape out the seeds and slice into ½-inch half-moons. Repeat with remaining squash.
In a large bowl, whisk together maple syrup, ¾ teaspoon salt, smoked paprika and 2 tablespoons olive oil, add squash slices and toss to coat evenly. Transfer to prepared baking sheet, spreading into a nearly single layer. Roast, tossing occasionally, until tender and golden brown, 30 to 35 minutes. (Don't roast so long that the squash lose their shape.)
Meanwhile, in a small bowl, whisk together yogurt, lemon juice, tarragon, remaining ¾ teaspoon salt and garlic. In a steady stream, whisk in remaining 4 tablespoons olive oil; whisk until emulsified.
In a large bowl, combine radicchio, arugula, squash, walnuts and green onions. Pour in dressing and toss to coat; season to taste with salt and pepper. Serve.