Heat 2 cups of water in a medium saucepan over high heat and bring to a boil.
Remove the saucepan from the heat, add the dried chile de arbol peppers, cover, and let sit and soak for 15 minutes to soften and rehydrate.
While the chiles soak, place the tomatoes on a large baking sheet. Broil in the oven for 8-10 minutes, flipping the tomatoes halfway through roasting, until the skins are blistered and charred.
Transfer the softened chiles to a blender using a slotted spoon to drain out any water.
Add the roasted tomatoes, garlic, white onion, vinegar, salt, cumin, and oregano. Puree until smooth.
Heat olive oil in a medium saucepan over medium-high heat. Add pureed sauce, stir to cook, and bring to a simmer. Remove from heat.
Assemble the sandwiches by filling each roll with pork carnitas, red onions, and a sprinkle of cotija cheese.
Place each sandwich in a shallow bowl or a plate with a lifted edges. Pour a generous amount of sauce all over the top of each sandwich.
Squeeze a lime wedge or two on each sandwich and serve.
Use a crusty bread. It ensures that the bread doesn’t totally disintegrate and fall apart after soaking and sitting in the sauce. A crusty French baguette or a sourdough are great options!
Adjust the heat level of the sauce to your liking. 8-10 peppers gives the sauce a medium heat. For a mild heat, use 5 peppers. For an extra spicy kick, use 15 or more peppers.
Strain the sauce for an extra smooth consistency if desired. The sauce is typically strained before serving to achieve a smoother consistency (almost like a hot sauce), but this step is optional. I personally prefer the sauce when it's not strained since I find that it sticks to the bread better, but it's totally up to you