Combine the tomatoes, garlic and chiles de árbol in a medium saucepan, cover with water and bring to a boil over medium-high heat. Reduce heat to medium and cook for about 10 minutes, until the tomatoes and garlic are completely soft and the chiles are plump and rehydrated. Transfer to a blender with 1 cup of the cooking liquid. Add the oregano, tomato paste and salt, and puree until completely smooth.
Heat the 1 tablespoon of the oil in a medium saucepan over medium heat. Once hot, pour in the tomato puree, cover partially and cook, stirring occasionally, for 5 to 6 minutes, until the mixture has thickened and darkened, and the flavors have intensified. Turn off the heat.
Season the butterflied shrimp with salt and pepper. Melt 1 tablespoon butter with 1 tablespoon oil in a large sauté pan over high heat. Once the butter begins to foam, add half the shrimp and quickly sear for a minute or so per side. They should be browned and just cooked through. Be careful not to overcook or the shrimp will be rubbery. Scrape into a bowl. Melt another tablespoon butter and oil together and repeat with the second batch.
Heat a comal, griddle or large skillet, preferably nonstick or cast iron, over medium heat. Add a tablespoon or two of oil to the surface.
One by one, lightly sauce the tortillas: briefly dip them into the tomato sauce, making sure the entire tortilla is coated (I like to use a pair of rubber tipped tongs but you could also just use you hands) and lay as many as will fit on the comal or griddle without overlapping. Top each tortilla with 2 to 3 tablespoons shredded cheese. Leave for a couple of minutes, until the cheese begins to melt and the bottoms of the sauced tortillas begin to dry and brown a little. Then, using a spatula, stack two sauced, browned and cheese-topped tortillas, one on top of another.
Don’t worry if the tortilla that you scrape up to stack on top of the other one sticks and tears a little bit or if it is not sitting evenly on top. Spoon some seared shrimp on top of each stack, gently fold with a spatula and continue cooking for a couple of minutes, until the cheese has completely melted and begun to ooze out and create a crust.
Remove from the heat and serve hot. Top with slices of ripe avocado.