The Kitchn: Skillet version of shepherd's pie cooks up quick
4 tablespoons unsalted butter
1 large egg yolk
For the filling:
1 teaspoon vegetable oil
1 1/2 pounds lean ground beef
1 teaspoon kosher salt
1 (10-ounce) bag frozen vegetable medley, such as carrots, peas, green beans, and corn
1/2 cup water
2 tablespoons all-purpose flour
2 tablespoons ketchup
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