Place couscous in a heatproof bowl and add 2 cups boiling water; mix, cover and let stand for at least 10 minutes. Reserve at room temperature.
Wash and dry lettuce; tear into bits and place into a large serving bowl or deep platter. Add walnuts, tomatoes, mushrooms, squash and gently toss.
In a bowl, whisk oil, vinegar, garlic, sugar, cinnamon, salt and pepper until very well-combined. Pour dressing over salad; add the couscous and sprinkle on the feta cheese. Gently mix. Serve at room temperature, or slightly chilled.