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.
As written, this recipe will give you undercooked leeks and nearly raw carrots. I sauteed the vegetables longer to make sure they were reasonably well cooked. Recipe also doesn't mention that pearl barley takes 1.25 hours to cook. End result was bland and extraordinarily mediocre for all the time it took. Don't bother.
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