Melt the butter in a skillet or saucepan over medium heat on the stove. 

Add the flour to the butter, and whisk to combine well, cooking for a minute or two until the raw flour taste has been cooked off.

Slowly whisk in first the milk and then the heavy whipping cream.

Add pepper to taste.

Bring the mixture to a rolling simmer over medium heat, and whisk often to prevent burning.

Once the sauce has thickened to your liking, add the beef into the skillet or saucepan, and stir to combine. Cook an additional 3-5 minutes and then serve over toast.