To make lentil ragout: In a small saucepan, soak lentils in cool water for 15 minutes. Drain, then cover with fresh water and cook over medium heat until soft but not mushy, about 15 minutes. In a separate pan over medium heat, sauté onion, celery, and garlic in olive oil for 5 minutes. Stir in thyme, tomatoes, and tomato paste. Add vegetable stock and bring to a boil. Reduce heat, cover, and simmer gently for 30-40 minutes. Stir in lentils; season with salt and pepper.