Step 1

Preheat the oven to 300 degrees F. Coat the top round with the garlic, onion and liberal amounts of salt and pepper. Rub the seasoning into the meat and place on a roasting rack in a baking pan. Roast until the internal temperature reaches 125 degrees F, about 1 hour. Remove from the oven and set aside to rest for at least 90 minutes before slicing.

Step 2

Put the ancho chiles and oil in a blender and blend on high speed until finely pureed. Strain through a fine-mesh strainer or cheesecloth into a container. Gradually stir chile oil into the mayonnaise until the desired spice level is reached; the mayonnaise should be a bright neon orange.

Step 3

Cut the flatbread into twelve 8-by-9-inch rectangles and toast with beef tallow on a griddle over high heat until light golden brown. Slice each rectangle in half horizontally, then smear the cut-sides with mayonnaise.

Step 4

On baking sheets, make twelve stacks of sliced roast beef, caramelized onions and Swiss cheese. Run under the broiler until the cheese is melted and the meat is warm.

Step 5

Transfer the stacks to the flatbread bottoms and form sandwiches. Slice the sandwiches on a diagonal to plate.

Step 6

Mix together the yeast, sugar and 1 1/2 cups of warm water (just under 110 degrees F) in a large bowl and set aside until tripled in size.

Step 7

Place the yeast mixture into the bowl of a stand mixer fitted with the dough hook and add the flour, oil, salt and 4 cups of tepid water. Mix on low speed until incorporated, then mix on medium speed for 10 minutes. Remove the hook but leave the dough in the mixer bowl. Let rise until doubled in size, about 30 minutes.

Step 8

Preheat the oven to 375 degrees F. Remove the dough from the bowl and divide into 4 pieces. Place each piece on an oiled baking sheet and let rest for 30 minutes. Press the dough out to fill the baking sheets, then let rest for another 20 minutes. Press the dough again to fill out the baking sheets; let rest for 20 more minutes. Press the dough for the third time to fill out the baking sheets; let rest for 5 minutes.

Step 9

Bake for 18 minutes, rotating the baking sheets halfway through.