Here is my version, which varies considerably from the initial recipe provided to me via the Internet. The main idea is that the Arby's® sauce is not very strong/spicy, it is sweet, and it tastes like ketchup is a significant component.
You might start with my recipe and decide if and how much you want to vary it to please yourself. You can easily scale down my recipe to make smaller quantities. The tamarind paste can be purchased via the Internet via sites like Amazon®.
For the beef you will need to make a fine hot sandwich, look at my recipe for Eye Roast of Beef shown at the bottom of this recipe.
Ingredients: (makes roughly one quart of sauce)
1 cup of Cattlemen's Kansas City Classic BBQ sauce
1 tsp. of chili powder
1/2 tsp. of tamarind paste
1 1/2 cups of ketchup
1 cup of water (or more)
1/4 cup of sugar
Directions:
Combine all ingredients in a small saucepan.
Cook over medium heat, stirring constantly, until the sauce boils.
Remove the sauce from the heat, pour it into a one quart canning jar, seal the lid and cool the sauce to room temperature.
Refrigerate the sauce. It will easily keep for a month or more.
Use the sauce generously on hot roast beef sandwiches. Directions for making the roast beef come next.
Here is the recipe for making the roast beef for your similar to Arbys® roast beef sandwich:
I buy roughly 3 lb. eye roasts of beef, and I cut them in half and vacuum seal and freeze the pieces until I am ready to use them, individually.
Later I thaw one piece and I remove any fat that might be present on a surface, and I stand it vertically on an aluminum foil covered baking sheet and bake it for 45 minutes at 350 degrees F. Then I check the internal temperature with an instant read thermometer pushed down into the center of the roast from the top. The goal is 130 degrees F, and further baking will likely need to be done to achieve that temperature.
Wrap the beef in the aluminum foil from the baking sheet to maintain surface moisture, then continue baking. Check the temperature every five minutes so you don't overshoot the mark. Once the temperature reaches 130 degrees F remove the foil wrapped beef from the oven and let it sit, wrapped, on a wood cutting board for about 15 minutes.
I process the hot roast beef with my meat slicer to create very thin or shaved slices. Then I immediately vacuum seal ¼ lb. quantities in small vacuum sealing bags and I simply refrigerate the product. It is medium rare roast beef. It is tender and juicy and delicious. It has a proven shelf life of at least four weeks. And my cost was (in 2023) $3.99 per pound. In other words, it is idiotic to buy premium roast beef at the delicatessen for $12 or more per pound or to buy cheaper brands and have them spoil in your refrigerator within a week.
Why the ¼ lb. quantities? Because that is the amount you are likely to use to make one nice hot roast beef sandwich, so I am being economical in opening a package and using all the contents while they are fresh. I do not have to worry about any amount of the product spoiling in the refrigerator following opening the vacuum sealed package.
Enjoy!