For the mashed potatoes: follow your package instructions for thawing your mashed potatoes in the microwave. Typically put a certain number of mashed potato medallions in a microwave safe bowl, cover with syran wrap and microwave on high for 3 minutes. Carefully remove the wrap and mash with a fork as much as you can. Re-cover and put back in the microwave for another 3 minutes. Voila! Repeat this process until you have about 18 oz. or 1.5 bags of Trader Joes frozen mashed potatoes now warm and mased. Set aside.
While you're microwaving your mashed potatoes, prep your other ingredients: chop onion and parsley.
Preheat the oven to 400 degrees F.
In a large oven safe pan (I prefer my cast-iron), heat the oil over medium high heat. Once hot, add the chopped onions and dried rosemary and cook (stirring regularly) for 3-5 minutes.
Add the ground beef to the onion mixture and season with salt and pepper. Cook the ground beef, breaking it up as you go, until the meat is cooked through (about 5 minutes).
Add the tomato paste, flour and vinegar and cook for an additional 2 minutes.
Add the chicken stock and the frozen peas and carrots and bring to a boil. Season with salt and pepper again and add in the chopped parsley.
Turn off the heat and add the mashed potatoes on top of the ground beef and vegetable mixture. Drizzle with the melted butter and sprinkle the Parmesean on top.
Bake in the oven until the top is lightly browned (about 30 minutes). Garnish with additional parsley!