In saute pan over medium-high heat, add Italian sausage. Cook and crumble until no longer pink.
Add potatoes, mushrooms, garlic powder, Italian seasoning and red pepper flakes.
Using a slotted spoon, add sausage to slow cooker, leaving as much grease in the pan.
Return pan to heat and add onions, celery, salt and pepper. Cook for 3 minutes. With a slotted spoon, add onions and celery to the slow cooker leaving as much grease in the pan. Discard grease.
Pour chicken broth into slow cooker and stir to combine. Secure the lid and cook for 5 hours on High (or 8 hours on Low).
In the last 30 minutes of cook time, ladle out as much of the broth as you can into a bowl.
In sauce pan over medium heat, add butter. Once melted, whisk in flour and cook for 3 minutes. Ladle in broth, about a cup at a time, and whisk into rue. Once all the broth is added, add heavy cream and whisk together. Let cook another 3 minutes. Then add gravy back to slow cooker. Add in fresh spinach and stir to combine. Re-secure the lid and let cook for remainder of cook time.
Serve in a shallow or soup bowl.