In a large saucepan filled with a steamer rack placed at least 1 inch above the boiling water, place the squash flesh side down. Cover and steam until the squash is fork-tender but not mushy, about 15 minutes.
Cool the squash slightly. To release the spaghetti strands, drag a fork from end to end as you would a rake and place in a serving bowl.
In a small bowl, whisk together the orange zest, juice, mustard, olive oil, sage, and salt and pepper. Toss with the squash and garnish with the sage leaves.