In a medium saucepan, combine lentils and vegetable broth. Bring to a boil, then reduce heat and simmer for about 20–25 minutes, or until the lentils are tender but not mushy. Drain any excess liquid and set aside.

Heat olive oil in a large skillet over medium heat. Add the chopped onion and cook until translucent, about 5 minutes. Add garlic and sauté for another minute until fragrant.

Add the diced carrot and celery to the skillet, cooking for about 5 minutes until they begin to soften. Stir in the sliced wild mushrooms and cook until they release their moisture and begin to brown, around 8–10 minutes.

Add tomato paste, soy sauce (or tamari), thyme, and rosemary to the skillet, stirring to coat the vegetables. Season with salt and pepper. Mix in the cooked lentils, then add the frozen peas and corn if using. Simmer for 5 minutes, allowing the flavors to meld together.

While the filling simmers, boil the peeled and cubed potatoes in a large pot of salted water until tender, about 15–20 minutes. Drain and return the potatoes to the pot. Add milk and butter, mashing until smooth and creamy. Season with salt and pepper to taste.

Preheat the oven to 400°F (200°C). Transfer the lentil and mushroom mixture to a baking dish, spreading it out evenly. Spoon the mashed potatoes over the filling, spreading them out to cover the top. Use a fork to create texture on the mashed potatoes, which will crisp up in the oven.

Bake for 20–25 minutes or until the top is golden and slightly crispy. For an extra golden crust, broil for an additional 2–3 minutes. Garnish with chopped parsley or chives, if desired. Serve warm and enjoy!