In a large pot over medium-high heat, add about 2 teaspoons of olive oil; add the ground sausage, breaking it up with a wooden spoon.
Cook the sausage until it is mostly browned, about 7-10 minutes. Remove the sausage from the pot and place on a separate plate; set aside. (You may need to drain some of the grease if you use pork sausage, which contains more fat.)
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.
Using an immersion blender, puree the mixture until completely smooth. (Or leave some larger pieces if desired.)
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!
Stir in the half and half or cream, cooked sausage and spinach. Heat until the spinach is just wilted, about 5 minutes.
Season with salt and pepper to taste. (Be cautious as sausage is often salty.)