Heat the olive oil in a spacious pot or Dutch oven over medium heat. Add the diced onion, minced garlic, and celery, and sauté until fragrant and softened, about 5 minutes.
Stir in the chopped carrots, thyme, bay leaves, and smoked paprika. Let the mixture cook for 2-3 minutes to release the flavors. Add the tomato paste and stir well, allowing it to caramelize slightly.
Add the white beans, lentils (if using), and sliced vegan sausage to the pot. Add the vegetable stock and mix to combine. Season with salt and black pepper to taste.
Cover the pot and simmer on low heat for 45 minutes to an hour, stirring occasionally, until the flavors meld together beautifully.
If you’d like a crispy topping, preheat your oven to 375°F (190°C). Transfer the cassoulet to an oven-safe dish. Mix the breadcrumbs with olive oil and sprinkle over the cassoulet. Cook for 10-15 minutes, or until they turn golden and crisp.
Serve it warm alongside crusty bread or a fresh side salad for a well-rounded meal.