Remove beef slices from jar and separate. Rinse under warm water. Place beef
on paper toweling in a single layer to drain. Pat dry with more paper towels. Re-stack
slices and chop into bite-size pieces.
In a large skillet, melt butter over medium heat. Reduce heat to low, add flour
and stir. Add beef, stirring to coat. Slowly add milk, stirring after each addition
until mixture is smooth.
Increase heat to medium-high and cook until mixture begins to boil and thickens.