1. Heat the olive oil in a large wide pan over medium heat. Add the onion, carrots, celery and a pinch of salt and freshly ground pepper. Cook for 10 minutes until vegetables are soft and tender.
2. Stir in the garlic, lentils, vegetable broth, bay leaves, thyme, rosemary and bring to a boil. Reduce heat to a low, cover and simmer for 30-35 minutes until lentils are tender. Stir every 5 minutes or so, making sure vegetables aren't sticking to the bottom of the pan. You may need to add more vegetable broth. Once lentils are cooked, whisk in the tomato paste.
3. While the lentils are cooking, make the potatoes. Place the potatoes in a medium saucepan and cover by 1-inch with water. Stir in the salt and bring water to a boil. Reduce heat to medium and simmer potatoes until they are fork tender, about 15 minutes.
4. Drain the water from the potatoes and pass them while they are still hot through a food mill or ricer. Place the potatoes back in the medium saucepan and stir in the milk and melted butter. Season to taste with salt and freshly ground pepper.
5. Remove the bay leaves, thyme bundle, rosemary and discard from the lentil mixture and stir in the peas.
6. Preheat the broiler. Transfer the lentil mixture to a 9x13 baking dish. Spread the mashed potatoes over the lentils in an even layer. Place the baking dish under the preheated broiler and broil until the potatoes are golden brown and crispy, about 5 minutes.
7. Remove from oven, garnish with parsley and serve.