Place whole squash on pan and bake, uncovered, at 375° for 30-40 minutes or until tender.
When cool enough to handle, cut squash in half then scoop out squash seeds and discard. Scoop out squash separating strands with a fork. Place in a colander and lightly sprinkle with sea salt to remove excess moisture.
Meanwhile, cook onion, garlic and red pepper flakes in oil in a heavy skillet over moderate heat, stirring, until garlic is golden, about 1 minute.
Add broccoli, lemon peel, salt and pepper and cook, breaking up frozen chunks and stirring occasionally, until broccoli is thawed and crisp tender, 7 to 10 minutes. Stir in chickpeas and cook until heated through. Add reserved spaghetti squash last and stir to combine.
Place in serving bowls. Top with crushed walnut mixture.