Slice the Italian sausage into bite-sized pieces. Clean and chop the leeks, making sure to remove any grit. Dice the celery, carrots, and bell peppers. Mince the garlic and fresh thyme. Open your cans of stewed tomatoes and have your lentils measured out and rinsed if needed.
Heat half of the olive oil in your large pot over medium heat. Add the sliced sausage and cook until browned on all sides, about 5–7 minutes. Remove the sausage with a slotted spoon and set aside, leaving the flavorful drippings in the pot.
Add the remaining olive oil to the pot. Toss in the onions, leeks, celery, carrots, and bell peppers. Sauté for 7–10 minutes until the vegetables have softened and the onions turn translucent. Stir in the minced garlic, thyme, and cumin, cooking for another minute until fragrant.
Return the browned sausage to the pot. Pour in the lentils, stewed tomatoes (with juices), and chicken broth. If using, add the Parmesan rind now. Season with kosher salt and freshly ground black pepper.
Bring the soup to a boil, then reduce the heat to low and cover. Let it simmer gently for 35–45 minutes, stirring occasionally, until the lentils are tender but still hold their shape. Remove the Parmesan rind before serving.
Give your soup a final taste and adjust the seasoning as needed – more salt, pepper, or even a pinch of red pepper flakes if you like a little heat.