Preheat the oven to 400°F. Line a baking sheet with parchment paper or foil and coat with 1 tablespoon of the olive oil. Spread the sweet potatoes on the baking sheet in a single layer; bake until soft, about 30 minutes.

Meanwhile, cook the lentils. If using bacon: In a large soup pot over medium-low heat, warm 1 tablespoon oil. Add the bacon and cook, stirring, until it's crisp and most of the fat has rendered, about 10 minutes. Using a slotted spoon, remove the bacon and set aside. If not using bacon: In a large soup pot over medium-low heat, warm 3 tablespoons oil.

Add the leeks, onion, garlic, and paprika to the pot, stirring to coat. Season with salt, reduce the heat to low, and cook, partially covered, stirring often, until very soft, about 25 minutes. Add 1 tablespoon of water and stir, scraping up the bits on the bottom and sides of the pot.

Make a bouquet garni by placing the thyme, parsley, peppercorns, and bay leaves in the center of a square of cheesecloth, then bring the ends together and tie securely with kitchen string.

Add the lentils, bouquet garni, and stock to the vegetables, then bring to a simmer and cook until the lentils are tender, about 30 minutes. Stir in the roasted sweet potatoes, and add more stock if needed. Cook for another 10 or so minutes.

Remove the bouquet garni. Add vinegar, salt, and pepper to taste. Serve garnished with the reserved bacon (if using) and cilantro leaves.