Preheat your oven to 400°F (200°C).

In a large pot, combine the green lentils and vegetable broth. Bring to a boil, then reduce heat and let simmer for 20-25 minutes until lentils are tender. Drain any excess liquid.

While lentils are cooking, heat olive oil in a large skillet over medium heat. Add diced onion, carrots, and celery and sauté for 5-7 minutes until vegetables are softened.

Add minced garlic and tomato paste to the skillet and cook for another 2 minutes.

Stir in the cooked lentils, frozen peas, frozen corn, dried thyme, and dried rosemary. Cook for an additional 5 minutes, stirring occasionally. Season with salt and pepper to taste.

Transfer the lentil and vegetable mixture to a baking dish and spread it out evenly.

Spread the prepared mashed potatoes over the top of the lentil mixture, smoothing it out to cover completely.

Bake in the preheated oven for 25-30 minutes, until the top is lightly browned and the filling is bubbling.

Let the shepherd's pie cool for a few minutes before serving.