Preheat your oven to 400°F (200°C).

Place the peeled and cubed potatoes in a large pot of salted water. Bring to a boil and cook for 15 minutes or until tender.

Drain the potatoes and mash them with the butter, milk, salt, and pepper until smooth. Set aside.

While potatoes cook, heat olive oil in a large skillet over medium heat. Add the onion and carrots, cooking for about 5 minutes until softened.

Add the sliced mushrooms to the skillet and cook for another 5-7 minutes until they release their moisture and brown.

Stir in the minced garlic, tomato paste, and dried thyme, cooking for 1 minute until fragrant.

Add the cooked lentils and vegetable broth. Simmer for 5 minutes to let the flavors meld and the sauce thicken slightly.

Stir in the frozen peas and season the mixture with salt and pepper to taste. Remove from heat.

Transfer the vegetable mixture to a 9x9 inch baking dish or casserole dish.

Spread the mashed potatoes evenly over the top of the vegetables. Use a fork to create ridges on the surface for crispy edges.

Bake for 20-25 minutes, or until the filling is bubbling and the potatoes are golden brown.