In a medium to large pot, sauté the onions in a few tablespoons of water until soft and translucent, about 5 to 7 minutes. Add the garlic and continue to sauté for another 2 minutes. Add water as needed, 1 tablespoon at a time, to prevent burning.
Add the squash, vegetable broth (just enough so that all the squash is in broth), rosemary, and red pepper flakes. Bring the mix to a boil, reduce to a simmer, partially cover and simmer for 15 to 20 minutes until the squash is tender.
Transfer the butternut/broth mix to a blender. Drain the walnuts, discard the soaking liquid, and add to the blender along with the lemon juice and salt. Blend on high until smooth. Preheat the oven to 375ºF if serving this like a casserole (see step 4).
Meanwhile bring a separate large pot of salted water to a boil and cook pasta according to directions. Once cooked, drain and rinse under cold water. Shake off any excess water and return the pasta to the pot. Add the butternut squash sauce to the pasta and mix well. Mix in the cooked chickpeas. You can eat this creamy pasta straight from the pot or you can transfer the mix to a 9x13 inch casserole dish and bake (in an oven preheated to 375ºF) uncovered for 25 minutes or until the top is browned.
When the pasta is done, remove from the oven and let sit for at least 5 minutes. Garnish with toasted walnuts and/or fresh rosemary. Divide among plates or bowls and enjoy!