Peel and chop your root vegetables into uniform chunks and place them in a large pot. Cover with water or vegetable broth and bring to a boil over high heat. Reduce heat to a simmer and cook until very tender, about 15-20 minutes.
Drain the cooked vegetables thoroughly, then transfer them to a bowl and mash until smooth or slightly chunky, depending on your preference. Mix in a tablespoon of butter or olive oil, a splash of milk or broth if desired, and season with salt and pepper to taste. Set aside.
While the vegetables cook, heat olive oil in a skillet over medium heat. Add diced onion and cook until golden and fragrant, about 5-7 minutes. Then add minced garlic and cook for another minute until aromatic.
Add chopped root vegetables to the skillet and stir to combine. Sprinkle with chopped thyme, salt, and pepper. Cook for about 10 minutes, stirring occasionally, until the vegetables are lightly caramelized and flavorful.
Preheat your oven to 200°C (400°F). Spread the sautéed vegetable mixture evenly in a 9x13 inch baking dish, creating a hearty filling with vibrant colors and fragrant aromas.
Dollop the mashed root vegetables over the filling, spreading gently to cover the entire surface. Use a fork to roughen the top for a rustic, textured look.
Bake uncovered in the preheated oven for 25-30 minutes, until bubbling around the edges and the top is golden and slightly crispy. For extra crispness, broil for 2-3 minutes, watching carefully.
Remove from the oven and let rest for at least 10 minutes to allow the layers to set and make slicing easier. The top should be crackly and fragrant, inviting you to serve.
Slice into hearty portions and serve hot, garnished with fresh herbs if desired. The dish should have a crispy top, tender filling, and a warm, earthy aroma that wraps you in comfort.
