In a large skillet over medium-high heat, brown the ground beef and diced onion until meat is no longer pink.
Drain most of the grease, leaving approximately 2 tablespoons in the skillet with the meat.
Sprinkle the flour over the meat and stir constantly for 1-2 minutes to cook out the raw flour taste.
Slowly pour in the milk while stirring continuously to prevent lumps.
Add salt, black pepper, and Worcestershire sauce.
Reduce heat to medium-low and simmer, stirring frequently, until the gravy has thickened to your desired consistency.
Adjust seasoning to taste and serve immediately.