Boil potatoes in a medium pot of salted water until cooked and soft.
Drain and mash with chicken broth, sour cream, 1/2 tsp salt and pepper, set aside.
Meanwhile, heat a large deep skillet over high heat and brown the meat, season with 1 teaspoon salt and pepper to taste. Cook, breaking the meat up with a wooden spoon as it cooks, about 5 minutes. When cooked, set aside on a plate.
Reduce the heat to medium-low and add the oil, onion, garlic, mushrooms and celery and sauté on medium heat about 5 to 7 minutes, until the vegetables are tender.
Add the frozen vegetables, tomato paste, Worcestershire sauce, rosemary, thyme, cooked meat and mix well. Whisk together the beef broth and flour then pour over the meat. Cover and simmer on low about 8-10 minutes, until the meat is tender and thickened.
Spread the meat on the bottom of casserole dish. Top with mashed potatoes and sprinkle with paprika.
Bake 20 to 25 minutes, then place under the broiler 1 to 2 minutes until the potatoes turn golden.
Remove from oven and let it cool 5 minutes before serving.