01 - Heat olive oil in a large heavy-bottomed pot over medium heat. Add the Italian sausage and cook until browned, approximately 5–6 minutes. Drain excess fat if necessary.
02 - Incorporate the diced onion, carrots, and celery into the pot. Sauté until softened, about 5 minutes.
03 - Stir in the minced garlic and cook until fragrant, approximately 1 minute.
04 - Add the rinsed lentils, diced tomatoes with their juice, broth, dried oregano, dried thyme, bay leaf, and optional crushed red pepper flakes. Stir to ensure all ingredients are well combined.
05 - Bring the mixture to a boil, then reduce the heat to low. Cover and allow to simmer for 30–35 minutes, or until the lentils reach a tender consistency.
06 - If using, stir in the chopped spinach or kale and cook for an additional 2–3 minutes until wilted.
07 - Season the stew with salt and freshly ground black pepper to your preference. Remember to remove and discard the bay leaf before serving.
08 - Serve the stew hot. Optional garnishes include grated Parmesan cheese and fresh herbs for an enhanced presentation.