Pickled onions: slice a large white onion through the stem/root. Remove peel on one half and thinly slice. Save the other half of the onion for the black bean spread. 

Add your thin slices of onion to a bowl with the white vinegar. Sprinkle with 1 teaspoon of sugar and 1/2 teaspoon of kosher salt. 

Stir everything together and leave on your counter for at least an hour, stirring occasionally.  At first, all the onions will not be inside the liquid, but as things soften and the onion releases more liquid, everything will eventually become submerged.

Add to a container with a lid and store in the refrigerator for a couple of weeks. 

Black bean spread: drain your black beans and discard all the juice. 

Add bacon fat or vegetable oil to a medium skillet over medium heat. When the fat is melted and shimmering, add diced onion.

Saute onions until they become slightly soft. Season your onion with salt and pepper at this point.

Add minced garlic and drained black beans. Cook over medium heat, stirring occasionally for 4 to 5 minutes. 

Remove from heat and allow the bean mixture to cool. Taste and add extra salt and pepper if needed. 

Add cooled bean mixture to a blender or immersion blender and blend until you get a spreadable consistency. If you do not have a blender you can use a potato masher. 

Store black bean spread in the refrigerator for up to a week. 

Arbol chili sauce: add one teaspoon of olive oil to a medium pan over medium heat. When the oil starts shimmering, add 4 minced garlic cloves and cook the garlic for 2 minutes, stirring often. Remove the garlic to a plate and wipe out the pan. 

Add 8 to 10 stemmed, dry arbol chiles to the pan still over medium heat and toast the chilis for 2 to 3 minutes. Remove the chilis to the same plate with the garlic.

Add 1/3 of the tomatoes from the fire-roasted tomato can to a blender. Add the garlic and toasted arbol chilies to the blender with 3 tablespoons of white vinegar, 1/4 cup of water and 1 teaspoon of salt. Blend until you have a fairly smooth consistency. This is your arbol chili salsa/sauce.

The brothy tomato sauce: add 1 teaspoon of olive oil to a medium pan over medium heat. Once the oil starts shimmering, add 1/2 large white onion, thinly sliced, and cook for about 4 minutes or until the onions start to soften a little. 

Add 4 minced garlic cloves to the pan with the onions and cook the garlic for 2 minutes, stirring often. After 2 minutes remove the garlic and onions from the heat. 

Add the other 2/3rds of the fire-roasted tomatoes, juice, and all to a blender. Add the onions and garlic to the blender with the tomatoes and blend until the mixture is smooth. 

In a large pot, add the blended tomatoes, onion and garlic and place over medium-high heat. Add 4 cups of chicken or vegetable broth, cumin, oregano, and salt, and bring to a boil. Allow the liquid to stay at a slow boil or simmer and partially cover the pot. Simmer for 10 to 15 minutes. This is your brothy tomato sauce. 

Torta ahogada assembly: slice a crusty six-inch sandwich roll and spread some black bean spread on the bottom roll. 

Top the black bean spread with a pile of pulled rotisserie chicken and top the chicken with a few pickled onions. Close the sandwich with the top roll. 

Traditionally the sandwich is placed in a bowl or large plate and sauce is poured over the top of the sandwich. Alternatively, you could dunk the sandwich in the sauce if you have a large enough pot. You can do either of these methods, but the slightly neater method below is the serving method that I prefer. 

Add about 1/2 to 1 cup of brothy tomato sauce to a soup bowl. Stir in a teaspoon of the arbol chili salsa (but be careful this is fairly spicy). Taste the broth to make sure it's not too spicy for your tastes. Add more arbol salsa if desired. 

Slice the torta in half and serve the sandwich partially dunked in the spicy tomato broth.  Top the whole dish with chopped cilantro if desired. To eat the sandwich you simply pick half out of the sauce and bite the part that has been dunked. Continue and keep moistening the sandwich in the sauce as you finish eating the torta.