Boil potatoes and parsnip in water until soft, about 15 minutes, then drain. Add butter, milk, and mustard and mash until smooth. Season to taste with salt and pepper. Set aside.
Sauté onions and carrots in oil until onions are tender, set aside.
Brown beef until fully cooked, drain fat, then add in onions, carrots, garlic and wine. Fry mixture for a few minutes until wine is evaporated, then sprinkle flour over mixture and mix well. Add remaining ingredients, and season to taste with salt and pepper. Continue simmering until mixture thickens, 8-10 minutes.
Spoon into an 8x8 baking dish, or divide among 6 ramekins. Top with potato mixture, sprinkle with cheese if desired and a drizzle of olive oil, and broil for a few minutes until tops are golden.
Notes
Large parsnips have a tough woody center. Quarter the parsnip, then slice out the center from each quarter (discard cores).