Guacamole-Remove skin from avocado. Dice up. Toss with lime juice, salt & sugar and mash. Beef-Marinate beef strips.Heat oil in pan.Add strips & cook till tender. Sauce-Heat butter and milk.Add flour and whisk in till smooth.Heat and allow to thicken till a pouring sauce forms.Add salt to taste. Then both cheeses.Mix into sauce. Assembly-Spoon guacamole on Nacho.Place on a beef strip.Top with tomato half.Drizzle sauce over before serving.