Place the dutch oven on the stovetop on medium-high heat. Add your sausage to the pot and sear for 2-3 minutes untouched. Break apart the sausage until browned, about 5 minutes. Use a slotted spoon to remove the sausage and set it aside.
Lower the heat to medium. Add 2 tablespoon of olive oil to the pot. Stir in the celery and leeks. Saute for 5 minutes or until softened. Stir in the oregano, parsley, pepper, and crushed red pepper.
Stir in the mushrooms, carrots, and red pepper. Saute for another 5 minutes until the mushrooms begin to darken or brown. Add the tomato paste and stir it into the vegetables. Allow the vegetable mix to sit for 3 minutes to caramelize the tomato paste.
Pour the chicken broth into the pot. Add the bay leaves and bring to a boil. Lower the soup to a simmer and return the sausage to the soup. Simmer for 15 minutes. Stir in the lentils and simmer for 25 minutes.
Add the spinach to the pot and cover for 3 minutes. Remove the cover and add the lemon juice. Stir and serve.
Notes
Sear the sausage to get a brown crust on the sausage and allow a fond to develop on the pot. This will increase the flavor of the soup and make it much more savory.
When you add the tomato paste, it's okay if it starts browning at the bottom of the pot. This caramelization of the tomato will intensify the tomato flavor of the soup.
Adding lemon juice at the end will brighten up the flavor of the soup.
Add a parmesan rind when you add the chicken broth for extra salty and nutty flavor.
Salt throughout the cooking process and taste your soup often.