In skillet, brown ground beef into crumbles, drain and set aside.
Wipe out same skillet, reserving a tiny bit of the burger fat and sauté mushrooms and onions for 3 to 5 minutes or until softened. Stir in Worcestershire sauce and set aside.
In large glass measuring cup, warm milk in microwave for 2 to 3 minutes on medium power (without boiling), stirring occasionally.
At the same time, heat butter in a large, heavy-bottomed saucepan over medium heat until melted. Add flour to melted butter and whisk continuously, cooking until it is bubbling and has turned lightly golden brown, 3 to 4 minutes.
Stream in heated milk, whisking continuously, and continue to cook over medium heat until mixture bubbles and thickens slightly. Add cheeses and whisk well until melted smooth and incorporated. Next mix in mushrooms and onions (including any juices) and cooked ground beef. Season with pepper and truffle salt.
Pour into serving bowl, garnish as desired, and serve with chips, veggies or crusty bread chunks. Also good leftover heated and stirred into pasta or scooped into tortillas.