Preheat the oven to 450°. In a large saucepan, heat 1 tablespoon of the oil. Add the bacon and cook over moderate heat until golden brown, about 4 minutes. Add the onion and cook, stirring, for 3 minutes. Add the stock, whole garlic cloves, carrot, leek, bell pepper, lentils and thyme and bring to a boil. Simmer over low heat for 30 minutes.
Meanwhile, in a baking dish, drizzle the chicken with 1 tablespoon of oil and season with salt and pimentón. Roast on the top rack for about 40 minutes, until cooked through.
Discard the thyme sprig from the lentils. Transfer the garlic, carrot, leek and bell pepper to a blender. Add 1/4 cup of the cooking liquid and puree. Drain the lentils and return them to the pot. Stir in the pureed vegetables.
In a small skillet, heat the remaining 1/4 cup of oil. Add the chopped garlic and cook over low heat for 1 minute. Add 1 teaspoon of pimentón. Scrape the mixture into the lentils and simmer for 5 minutes. Add the vinegar and season with salt. Serve the lentils in bowls topped with the chicken.