Add onion and red bell pepper to the pot and cook for 3-4 minutes or until they are softened and fragrant. Add the minced garlic and cook for one more minute.
Stir in the diced squash, ground (or fresh) sage and red pepper flakes; cook for another 5 minutes or so, just to combine the flavors and begin to soften the squash.
Pour in chicken broth and water and give everything a stir. Bring the mixture to a boil and allow it to simmer for about 12-15 minutes or until the squash is very tender and easily pierced with a fork.
If you don't have an immersion blender, you can pour the mixture into a blender and blend in batches -- just be careful as blending hot liquids can cause the lid to pop right off!