Preheat oven to 400 degrees.
In a blender, combine tahini, olive oil, lemon juice and ½ teaspoon salt; blend until smooth.
Chop the tip and tail off the spaghetti squash, cut in half lengthwise, and scoop seeds out of each half. Rub a tablespoon of olive oil over flesh of each half. Sprinkle with sea salt and pepper. Place squash, cut-side down, on a baking sheet. Roast for 35 to 50 minutes, or until tender.
Meanwhile, heat 1 tablespoon olive oil in a large skillet. Add Brussels sprouts and cook, stirring occasionally, until they turn bright green and begin to soften, about 5 minutes.
Once squash has finished roasting, allow it to cool enough to handle, then use a fork to release the "spaghetti" strands. Add to skillet with Brussels sprouts, along with desired amount of sauce. Stir to combine and continue cooking until excess water has burned off. Season with sea salt and pepper, to taste.
Top with pumpkin seeds, if using, and serve.