Prepare the au jus dipping sauce: Melt butter in saucepan; saute onions until tender; add flour and stir; add contents of gravy mix packet then gradually whisk in water; allow to simmer while you make the sandwich.
Prepare the "secret" sauce: Stir together alfredo sauce, mayonnaise, and your choice of onion salt or garlic salt; slice french bread with one horizontal slice to make a top and a bottom half; spread "secret" sauce evenly over both pieces of sliced bread.
Place the prepared sandwich halves (open-faced) upon a baking sheet under the oven broiler for about 1 minute until the sauce is bubbly and the edges of the bread barely start to brown; remove from oven.
Arrange thin sliced roast beef over both sandwich halves then sprinkle evenly with shredded cheeses; broil for another minute (still open-faced) until cheese is bubbly all over and starting to brown; remove from oven.
Put sandwich halves together and return to oven to lightly toast (very, very briefly only) top and bottom of sandwich; remove from oven and slice sandwich into 5 even servings; provide 1/3 cup au jus with each serving.
NOTE: I usually make this sandwich using one large loaf of bakery french bread ~ You could go with smaller baguette buns if you want yours to taste AND look like the Quiznos Steakhouse Dip Sandwich.