Heat a large pot over medium/high heat. Add sausage and cook until browned, about 10 minutes. Remove sausage with a slotted spoon and add to a plate off to the side. Drain off half of the grease. Turn heat down to medium.

Add potatoes and sauté for about 10 minutes until they start to become slightly golden. At this time, add in the carrot, celery, and onion. Saute until the vegetables become tender.

Return the sausage to the pot along with the chicken stock, bay leaf, oregano, crushed red pepper flakes, and garlic powder. Stir to combine and bring to a simmer.

Add the lentils and cover with a lid. Let the stew simmer while covered for thirty minutes.

After thirty minutes, remove the lid, add in the slurry of flour and water. Double check there are no lumps in the slurry and that the flour is completely dissolved before adding. Stir to combine. Bring to a simmer again and cook for another 20 minutes until the soup thickens.

Add the spinach and heavy cream if you are using it. Stir and cook for another 5 minutes.

Remove the bay leaf, adjust salt and pepper to taste, and let the soup cool slightly before serving.