In a large Dutch oven or heavy bottomed stock pot, heat the butter over medium heat until melted. Add the onions and season with salt and pepper. Cook while stirring occasionally, until the onions are a deep caramel color, approximately 45-50 minutes.

Stir in the red wine vinegar in with onion mixture and scrape bottom of pot to dissolve small bits of browned food from the pot.

Add the FOND beef broth, thyme and bay leaves. Bring the mixture to a boil and then turn the heat to low. Simmer over low heat for 10 minutes to allow the flavors to combine. Taste and add more salt if desired.

Meanwhile, make the protein sparing bread. To make the protein bread baguette, preheat the oven to 325 degrees F. Place a piece of parchment onto a rimmed baking sheet and spray with Avocado oil spray and set aside.

Separate the eggs (save the yolks for another recipe like my keto ice cream, hollandaise, mayo or lemon curd), and place the whites into a large bowl or stand mixer. Add the allulose, salt and cream of tartar if using and whip the whites for a few minutes until VERY stiff.

Turn the mixer to low and gently mix the egg white protein powder into the whites. Do not over mix or the hot dog buns will end up like Styrofoam.

Spread the protein sparing bread mixture onto the prepared pan in a baguette shape that is about 3 inches round and 12 inches long. Bake for 15 minutes or until cook through and golden brown. Allow to cool completely before slicing. NOTE: The protein bread baguette can be made 4 days ahead and stored in the fridge (or a month ahead and stored in the freezer).

Slice the protein sparing baguette into 1/2 inch slices. Set aside.

Ladle the soup into four oven-safe crocks or bowls. Top each bowl with one slice of protein sparing bread and 1/4 cup of cheese. Broil the soup under the broiler on low until the cheese is melted and golden. Serve immediately.