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Heat oil in a 12-inch cast iron pan over medium-high heat. Add ground beef, onion, and mushrooms. Break ground beef apart as it cooks and cook until beef is browned and onion and mushrooms are soft, about 5 minutes.
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Add garlic, salt, pepper, and thyme and cook 1 minute.
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Stir flour into mixture and cook 1 minute.
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Stir in tomatoes, condensed French onion soup, beef broth, parsley, vinegar, and garlic powder.
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Simmer for 10 minutes, stirring occasionally.
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Meanwhile, peel potatoes and cut into 1 1/2-inch chunks. Place in a pot and cover with water. Add salt to the water ( a few teaspoons) and bring to a boil. Simmer until potatoes are soft.
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Drain potatoes. add butter, milk, and cheese. Mash with a potato masher until smooth. Season to taste with salt and pepper.
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Spread potatoes on top of beef mixture. Transfer pan to oven and broil for about 3 to 5 minutes.
