Preheat oven to 400. Pierce a spaghetti squash 8-10 times with a skewer or thin knife (pierce deeply through flesh into center). Place on a baking sheet on center rack of oven. Bake for 30 minutes and then turn 1/4 turn. Bake another 30 minutes or until outside has browned in places and shell feels soft. Remove from oven and set aside until cool enough to handle. Once it’s cool, cut it in half and remove and discard the seeds and scrape the strands of squash out with a fork. Put the squash into a bowl and set aside.
While the squash is cooking, prepare the Brussels sprouts. Trim and discard the ends and cut the sprouts in half (quarter larger sprouts). Place on a baking sheet and spray quickly (2 seconds) with olive oil. (This prevents burning; if you don’t want to use the oil, cover loosely with a sheet of aluminum foil.) When the squash has finished cooking, put the sprouts into the oven and bake for about 15 minutes, stirring halfway through. Remove them when they are just beginning to be touched by brown but are not burning (they will finish cooking in the skillet in the next step).
In a large, deep, non-stick skillet, cook the onions on medium-high heat until they become golden, about 5-6 minutes. Add the Brussels sprouts, garlic, and vegetable broth and cover tightly. Cook for 3-5 minutes, adding more broth or water if skillet becomes dry. Add the chickpeas, basil, and red pepper flakes. Stir in the spaghetti squash, and toss gently to mix. Cook until heated through. Add salt and pepper to taste, along with lemon juice. Serve topped with crushed or sliced almonds, if desired.