Place the cubed potatoes in a large saucepan and cover with cold, salted water. Bring water to a boil, and cook until potatoes are soft and tender, about 10-15 minutes. Drain and reurn the saucepan. Add in butter, milk, and sour cream, then mash well with a potato masher or fork. Season with salt as desired. Set aside.

Heat olive oil in a medium skillet over medium high heat. Add in ground beef, and cook until mostly browned, about 5 minutes. Season with salt and pepper

Stir in the onions, garlic, and mushrooms, and cook until mushrooms are tender, about 5-7 minutes.

Add in the frozen vegetables, beef broth, tomato paste, Worcestershire sauce, flour, and thyme. Mix well, and let simmer on medium-low heat for about 10 minutes, or until sauce is reduced to a thick, meaty gravy. Season with salt and pepper as desired.

While meat mixture is simmering, preheat oven to 400 degrees.

Spread meat and vegetable mixture along the bottom of a casserole dish (9 x 13 or oval shaped). Top with the prepared mashed potatoes, and sprinkle with a light dusting of paprika or cayenne, if desired.

Place in oven and bake for about 20-25 minutes. Let cool for at least 10 minutes before servings.