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.
I love sage, I like spaghetti squash. However, the combination of these two things and orange did not do it for me. Kind of made me feel ill. I used a cara cara orange so perhaps a more typical orange would have made it taste sweeter and maybe a little bit more palatable.