Preheat the oven to 425 degrees F. Heat a large skillet over high heat for 2 minutes, then add the oil. (It will shimmer and lightly smoke; that's OK.) When you start to see wisps of smoke, crumble the ground beef evenly into the pan. Let the beef cook undisturbed until browned on one side, 3 minutes, then start breaking it up and turning it to brown any remaining pink spots, about 4 minutes more. Transfer the beef with a slotted spoon to a medium bowl; set aside.
Turn the heat down to medium and discard all but 2 tablespoons of fat from the skillet. Add the onions, garlic and thyme, stir to combine and cook until the onions are soft and translucent, about 5 minutes. Add the flour and tomato paste and cook, stirring, for 2 minutes. Add the cooked beef, beef stock, Worcestershire sauce, soy sauce, 1 1/4 teaspoons salt and 1/4 teaspoon pepper and bring to a boil. Reduce the heat and simmer until the sauce has thickened, about 4 minutes. Remove the thyme sprigs if using. Fold in the peas and carrots.
Transfer the mixture to an 8-inch square casserole pan, and top with the potato tots and Cheddar. Bake until the potato tots are crisp and the meat layer is bubbling up the sides, 20 to 25 minutes.
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