Melt bacon fat or butter over medium heat. Add flour and cook for 2-3 mins. Whisk in milk, garlic, Worcestershire sauce, hot sauce and S&P. Cook until thickened and bubbly. Remove from heat and add 2/3 of the cheese and stir until melted.
Add onions and potatoes and mix. Pour into greased or sprayed 13x9 pan (I used glass). Sprinkle remaining cheese on top. Cover with foil and bake for 30 minutes. Remove foil and continue cooking until nicely browned and potatoes are done, approx. 30 minutes.