Rinse and sort the brown lentils, removing any small stones or debris. Dice the celery into 1/4-inch pieces, dice the onion into 1/2-inch pieces, cut the carrots into 1/4-inch rounds, and mince the garlic. Thaw the frozen spinach and squeeze out all excess moisture using a clean kitchen towel or paper towels—this prevents the soup from becoming watery. Having all ingredients prepped and ready will make the cooking process smooth and allow you to focus on building flavor.

Heat a large pot over medium-high heat. Remove the Italian sausage from its casing and add it to the pot, breaking it into small crumbles as it cooks. Brown the sausage for 5-7 minutes, stirring occasionally, until it's no longer pink and has developed some color. This initial browning creates a flavorful foundation for the entire soup and renders some of the fat to cook the aromatics in.

Add the diced onion, carrots, and celery to the browned sausage and cook for 5 minutes, stirring occasionally, until the vegetables begin to soften and release their aromas. Add the minced garlic and cook for another 1 minute. Now add the smoked paprika, ground cumin, cayenne pepper, and dried oregano, stirring constantly for about 30 seconds to bloom the spices and develop their deeper flavors. I like to toast the spices briefly like this because it intensifies their flavor—it's a technique that makes a real difference in the final taste of the soup.

Add the rinsed and sorted lentils and the chicken broth to the pot, stirring well to combine. Bring the mixture to a boil over medium-high heat, then reduce the heat to medium-low, cover the pot with a lid, and simmer gently for 30 minutes. The lentils should become tender and soft by the end of this time. Stirring occasionally helps ensure even cooking.

After the lentils are soft, stir in the thawed spinach that you squeezed dry in Step 1. Continue cooking, uncovered, for 15 minutes, stirring occasionally, to allow the flavors to meld and the soup to reach a nice consistency. Taste and adjust seasonings as needed with salt and pepper. I find that letting the spinach cook into the soup for a few minutes rather than adding it at the very end gives it a more integrated, cohesive flavor throughout the pot.