Preheat your oven to 200°C (390°F) and gather your baking dish, large skillet, saucepan, and a potato masher.
Peel and chop the potatoes into chunks, then boil them in salted water until they’re tender and easily pierced, about 15-20 minutes. Drain well and set aside.
Meanwhile, chop carrots into bite-sized pieces, slice the mushrooms, and dice the onion. In the large skillet, heat a splash of oil over medium heat and sauté the vegetables for about 8 minutes, stirring occasionally, until they soften and develop a golden hue. Add the minced garlic in the last minute, cooking until fragrant.
While the vegetables cook, mash the drained potatoes with butter and warm milk until smooth and creamy. Season with salt and pepper to taste, and set aside.
In the same skillet, sprinkle the flour over the cooked vegetables and stir well to coat. Gradually pour in the concentrated vegetable broth, stirring constantly, until the mixture thickens into a rich gravy. Add the chopped thyme, salt, and pepper, and simmer for another 2 minutes to deepen the flavors.
Spread the sautéed vegetable mixture evenly into your baking dish. Pour the thickened gravy over the vegetables, spreading gently to coat everything evenly.
Top the vegetable and gravy layer with the mashed potatoes, smoothing into an even layer with a spatula. You can create a rustic pattern with a fork for a charming, crispy surface.
Bake uncovered in the preheated oven for 25-30 minutes, until the top is golden and crispy around the edges, and the filling is bubbling around the sides. The aroma will be warm and savory.
Remove the pie from the oven and let it rest for 5-10 minutes. This helps the filling set and makes slicing easier.
Slice into squares, serve hot, and enjoy the comforting, veggie-forward flavors with a side salad or crusty bread.
