First, prepare the buns.
Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large bowl, whisk together the almond flour, psyllium husk powder, baking powder, and sea salt.
Add the egg whites and apple cider vinegar to the dry , followed by the boiling water.
Using a hand mixer or a sturdy whisk, mix thoroughly for about 30 seconds until the dough becomes thick and pliable.
Divide the dough into 6-8 equal portions and shape them into bun-like discs with wet hands to prevent sticking.
Place them on the prepared baking sheet.
Bake on the middle rack for 50-60 minutes, or until firm to the touch and golden brown.
Let them cool completely on a wire rack.
While the buns are baking, prepare the meat sauce.
Heat the olive oil in a large skillet or Dutch oven over medium-high heat.
Add the chopped onion and cook until softened, about 3-4 minutes.
Add the minced garlic and cook for another minute until fragrant.
Add the ground beef to the skillet, breaking it up with a spoon.
Cook until browned all over.
Drain any excess grease.
Stir in the crushed tomatoes, tomato paste, chili powder, ground mustard, red wine vinegar, salt, and pepper.
Bring the mixture to a boil, then reduce the heat to low.
Allow the sauce to simmer for at least 10-15 minutes, stirring occasionally, to allow the flavors to meld.
Taste and adjust seasonings if necessary.
To serve, slice the cooled buns in half. Spoon a generous portion of the hot meat sauce onto the bottom buns, top with shredded cheese and a dollop of mayonnaise, and place the top bun on.
After slicing the buns, lightly toast the cut sides in a dry pan or under the broiler for a minute to enhance their texture and sturdiness.
