Steps:

Preheat the oven to 400°F (200°C).
In a large pan, heat the olive oil over medium heat. Add the onion, carrots, and celery, and cook until soft.
Add the garlic, thyme, and rosemary, and cook for another minute.
Stir in the lentils, vegetable broth, and tomato paste. Bring to a boil, then reduce heat and simmer for 25-30 minutes, until lentils are tender.
In a small bowl, mix the flour with a little water to make a paste, then stir into the lentil mixture to thicken. Season with salt and pepper.
Spoon the lentil mixture into a baking dish. Top with the mashed potatoes and smooth the top.
Bake for 20 minutes, or until the top is golden. Let stand for 5 minutes before serving.

Lentil Cottage Pie