Preheat the oven to 450 degrees F. Cut the spaghetti squash length-wise and scoop out the seeds. Place the squash cut-side-up on a rimmed baking sheet. Roast in the oven for 35-45 minutes, until firm yet cooked enough to shred. Prep all other ingredients while waiting.
When the spaghetti squash is almost ready. Preheat a large skillet to medium-high heat. Pour the sesame seeds on a plate. Then salt and pepper the ahi steaks on both sides and press into the sesame seeds to coat. Add 1 teaspoon sesame oil to the skillet. Once hot, add the ahi steaks and sear for no more than 2 minutes per side. (I do mine for 90 seconds per side.) Allow the ahi to rest.
Shred the roasted spaghetti squash with a fork and place in a large mixing bowl. Add all remaining ingredients and toss to coat. Then while the spaghetti squash is still warm, arrange it on plates and slice the tuna steak into thin strips. Place the tuna over the top and serve!