Coat the beef eye of round with dijon mustard and season on all sides with the beef seasoning
Preheat your grill to 250 degrees and set up for indirect heat. Add a few chunks of wood for addtional smoke flavor. Smoke the beef roast for about 2 hours or until the internal temperature reaches 120 degrees. Wrap it in foil and let if rest for 45 minutes to 1 hour.
While the beef roast is resting, we can start on the cheese sauce. Melt the butter in a cast iron skillet, over medium direct heat. Add the flour and whisk for a minute or 2 to cook out the raw flour taste and smell.
Add the milk, velveeta cheese and continue mixing until the cheese is melted.
Add the shredded cheddar cheese, mix and cook until the sauce is silky smooth. Remove from the heat.
For the horsey sauce, in a mixing bowl, add white vinegar, sugar, mayo and horseradish. Mix the ingredients together and set aside.
Remove the beef roast from the foil wrap and slice as thinly as you can.
Assemble the sandwich on an onion roll, add the horsey sauce, sliced beef, cheese sauce, and enjoy!