Preheat Oven to 350 degrees
Place Potatoes in a large pot and boil for approximately 20 minutes, or until tender.
Mash or rice potatoes, add 2 tablespoons of butter and ½ cup of milk. Stir until smooth and keep warm (salt & pepper to taste).
Heat 1 tablespoon of butter over medium high heat in large cast iron skillet, add all mushrooms and cook, stirring frequently until juices are released from mushrooms and they begin to brown, approximately 6 to 8 minutes. Remove from pan and salt & pepper to taste.
Heat 1 tablespoon of butter over medium high heat in same skillet and add onions and celery, cook until softened, about 8 minutes. During last minute add garlic and stir. Add onion, garlic and celery to reserved mushrooms.
Heat 2 tablesppons of butter over medium high heat in the same skillet. Add root vegetables (parsnips, turnips and carrots) sauté until tender, approximately 8-10 minutes.
Add wine and deglaze pan by scraping any browned bits.
Add tomato paste, herbs (sage & thyme), vegetable broth and flour. Heat until slightly thickened, about 3 to 4 minutes.
Add peas and corn, cook until heated through, approximately 1-2 minutes.
Return reserved mushroom and vegetable mixture to pan and stir to combine. Remove from heat.
Spread mashed potatoes over vegetable mixture and texturize a bit to form peaks.
Sprinkle top with cheddar cheese.
Place in upper ⅓ of oven and bake until cheese melts & potatoes lightly brown, 15 to 20 minutes.
Serve hot and enjoy!