Cut squash in half lengthwise; discard seeds and stringy pulp. Place squash halves, cut sides up, on a 12-inch round glass platter; sprinkle with water. Cover loosely with plastic wrap; microwave at high 15 minutes or until tender, rotating platter a half-turn every 5 minutes. Let cool slightly. Scoop pulp from shells to equal 2 cups, reserving pulp and shells; set aside.
Heat oil in a large nonstick skillet over medium heat. Add mushrooms and next 4 ingredients (mushrooms through garlic); sauté 5 minutes. Stir in reserved squash pulp, spinach, and next 4 ingredients (spinach through red pepper). Spoon 1 cup spinach mixture into each squash shell half; sprinkle with sesame seeds.