Rinse the lentils under cold water. In a medium saucepan, add lentils and cover with water. Bring to a boil, reduce heat, and simmer for about 20-25 minutes, until tender but not mushy. Drain and set aside.

While the lentils are cooking, heat the olive oil in a large skillet over medium heat. Add the chopped onions and sauté until translucent, about 5-7 minutes. Stir in diced carrots and cook for another 5 minutes. Add garlic and cook for an additional minute until fragrant.

To the vegetable mixture, add the drained lentils, tomato paste, Worcestershire sauce, thyme, rosemary, salt, and pepper. Mix well and cook for about 10 minutes until everything is well combined and heated through. Add frozen peas just before removing from heat.

Boil the peeled and diced potatoes in salted water until tender, about 15-20 minutes. Drain and return potatoes to the pot. Mash with plant-based milk and season with salt and pepper to taste.

Preheat your oven to 400°F (200°C). In a baking dish, layer the lentil and vegetable mixture at the bottom. Top with the creamy mashed potatoes, spreading them out evenly.

Place the baking dish in the preheated oven and bake for 25-30 minutes, until the top is golden and crispy. Let cool for a few minutes before serving.