Step 1: Prepare Your Ingredients: Thoroughly rinse the red lentils under cold running water until the water runs clear. Set aside to drain. Chop the onion, carrots, and celery into small, even pieces. Slice the leek, discarding the tough dark green tops and the root end, then rinse thoroughly. If using, peel and dice the turnip/parsnip and potato into similar-sized pieces. Mince the garlic if using.

Step 2: Sauté the Aromatics: Heat the olive oil or plant-based butter in your large heavy-bottomed pot or Dutch oven over medium heat. Add the chopped onion, carrots, and celery. Sauté for 8-10 minutes, stirring occasionally, until the vegetables have softened and the onion is translucent. Add the sliced leeks (and minced garlic if using) to the pot. Continue to sauté for another 3-5 minutes until the leeks are tender. If using smoked paprika, add it now and stir for about 30 seconds until fragrant.

Step 3: Add Lentils, Broth, and Seasonings: Stir the rinsed red lentils into the pot with the sautéed vegetables. Pour in the vegetable broth. Add the bay leaf and dried thyme. Season with 1 teaspoon of salt and ½ teaspoon of freshly ground black pepper to start. You will adjust the seasoning later.

Step 4: Simmer to Perfection: Bring the soup to a boil over medium-high heat. Once boiling, reduce the heat to low, cover the pot, and let it simmer gently for 30-40 minutes. Stir occasionally to prevent the lentils from sticking to the bottom. The lentils should break down, and the soup will thicken significantly. If you're adding turnip/parsnip and potato, add them about 15-20 minutes into the simmering time, ensuring they have enough time to become tender.

Step 5: Check Consistency and Seasoning: After simmering, remove the bay leaf. Check the consistency of the soup. If you prefer a smoother soup, you can use an immersion blender to blend a portion of the soup directly in the pot. Alternatively, carefully transfer about 2-3 cups of the soup to a standard blender (being cautious with hot liquids) and then return it to the pot. For a truly rustic soup, you can leave it chunky. Taste the soup and adjust the seasoning as needed. If it's too thick, add a little more hot vegetable broth until it reaches your desired consistency.

Step 6: Serve and Garnish: Ladle the hot Scottish Lentil Soup into bowls. Garnish with fresh chopped parsley. Serve immediately with a slice of crusty bread or oatcakes for a truly authentic experience.