Place a rack in the center of the oven and preheat to 425 degrees F. Line a baking sheet with parchment paper.
Cut the squash at the bottom of its long neck, separating the squash into 2 pieces. Cut each piece lengthwise and scoop out the seeds from the bottom half. Cut each piece into 1/4-inch half-moons. Add the squash to a large bowl along with 2 tablespoons of the oil, 1 teaspoon salt and a couple grinds of black pepper. Toss to combine, and then arrange on the prepared baking sheet in an even layer.
Bake on the center rack until the squash is tender but not mushy (the squash might not brown and that's okay), about 30 minutes. Set aside to cool at room temperature.
Combine the Greek yogurt, orange juice, 1 tablespoon of the olive oil, 1 tablespoon of the lemon juice, 1 teaspoon salt and a couple grinds of black pepper in a medium bowl. Whisk to combine. Season with additional salt if needed.
Toss the arugula with the remaining 1 tablespoon olive oil, 1 tablespoon lemon juice, a pinch of salt and a couple grinds of pepper in a medium bowl.
To serve, divide the yogurt among 4 plates, spreading it out into a thin layer. Then top with the butternut squash and arugula. Sprinkle with the pomegranate seeds and sunflower seeds.