Preheat the oven to 350°F (176°C).
Mix together mayo, mustard, poppy seeds, grated onion, horseradish, and Worcestershire. Taste the dressing and adjust the ingredients however you wish.
To assemble, spread the sauce inside the top and bottom of each bun. On the bottom bun, layer provolone, roast beef, and American cheese, and place the top bun on top. Repeat for remaining portions.
Wrap each sandwich in aluminum foil, and put them on a baking sheet in the oven until the buns are slightly crusty and the cheese is melted, about 10 minutes.
Prepare Margaritas: Mix the ingredients for the margarita. If desired, rim the glasses with salt by rubbing a lime wedge around the rim of each glass, then dipping the rim into salt to coat.
Make the Queso: Mix the ingredients (except for the chips) in a saucepan over low heat. Stir occasionally, until the cheese is melted and creamy. Season with salt and pepper to taste. Cover with a lid to keep warm.
Serve the hot roast beef sandwiches with chips and queso and virgin margarita.