In a large pot or dutch oven over medium heat, heat olive oil. Add onion and cook until soft, about 6 minutes. Stir in zucchini, water and bouillon cube. Season with salt and pepper
Reduce heat to low and cover pot. Cook 4 hours, stirring often, until the zucchini is falling apart (it will be mushy, and that's good!). When sauce was done, I used my immersion blender to make it more "saucy" and less chunky.
What the sauce is almost ready, bring a large pot of salted water to a boil. Add pasta and cook according to package directions. Drain. Season sauce with salt and pepper, then stir in lemon juice, Parmesan and red pepper flakes. Serve immediately.