Add beef broth, Worcestershire sauce, salt, garlic powder, onion powder, Italian seasoning, and red pepper flakes to a pot on the stove. Bring the ingredients to a simmer. Then, add all the slices of roast beef to the simmering broth, one at a time. Thinly-sliced roast beef will be best for this recipe.
Stir and simmer the beef slices in the developing au jus for up to 10 minutes. Next, grab a large saute pan or skillet and melt the butter in it over low heat. Once the butter is melted, scoop all of your roast beef from the pot into the pan. Pour all of the remaining au jus into the pan but reserve about ⅓ of the liquid in the pot.
Bring the pan full of beef and au jus to a simmer. You’ll need to simmer the ingredients down until the butter and liquid emulsify – about 10 minutes or so. Wait for your reserved liquid to cool down before whisking tapioca flour into it to thicken it. After it thickens, reduce the heat on your pan of beef so the au jus is just hot – not bubbling. Pour and stir the thickened liquid into your pan of beef.
Let the pan of thickened au jus bubble over low heat and thicken further. Once you have a saucy skillet (not soupy), place slices of swiss cheese on top of the skillet. Let the cheese melt over low heat and then serve your French dip skillet hot. It’s best served with slices of toasted Keto bread for dipping