Melt 4 tablespoons unsalted butter in a medium frying pan over medium heat. Add 2 tablespoons all-purpose flour and stir constantly with a wooden spoon until a light caramel color, about 1 minute.
Reduce the heat to low. Add 1 cup water and stir until combined. Add 1/3 cup ketchup, 1/3 cup Worcestershire sauce, 2 tablespoons oyster sauce, 2 tablespoons soy sauce, 2 tablespoons packed light brown sugar, and 1 teaspoon kosher salt. Continue to simmer, stirring occasionally, until slightly thickened into a gravy-like consistency, 6 to 7 minutes.
Turn off the heat. Add 1 teaspoon lemon juice and stir until combined.
