Mashed Potatoes and Celeriac
Place the potatoes, celeriac and garlic cloves in a large pot. Cover with cold water and season generously with salt. Bring to a boil. Simmer for 15 to 20 minutes or until the vegetables are very tender. Reserve ½ cup (125 ml) of the potato cooking water. Drain the potatoes well and return to the pot.
Add half of the reserved cooking water along with the milk and butter to the pot of potatoes. Mash until very smooth. Add more cooking water or milk as needed. Stir in the Parmesan cheese until melted. Season with salt and pepper to taste. Keep warm.
Lentil Filling
Preheat the oven to 425°F (220°C).
In a large pot over medium heat, melt the butter. Add the carrots, mushrooms, shallots, garlic, thyme and sage. Season with salt and pepper. Cook for 5 minutes, stirring regularly. Add the flour and tomato paste. Cook, stirring, for 2 minutes. Pour in the wine, scraping the bottom of the pot with a wooden spoon. Cook for 2 minutes.
Add the lentils and stir to coat. Add the vegetable broth and bay leaves. Let simmer for 30 minutes or until the lentils are tender. Remove the bay leaves.
Stir in the balsamic vinegar and soy sauce. Mix well and adjust the seasoning. Add the peas and corn.
Transfer the lentil mixture to the Heritage Oval Casserole, spreading it out in an even layer. Top with an even layer of mashed potatoes. Use a fork to make a decorative pattern in the mashed potatoes.
Bake in the oven for 30 minutes until bubbling. Finish under the broiler, if desired.