Put the tenderloin on a parchment-lined baking sheet and freeze for 1 hour. Flip the tenderloin and freeze for another hour. Use a very sharp knife to slice the meat as thinly as possible.
Roast the poblano directly over an open flame, turning regularly, until blistered and blackened all over, about 5 minutes. Put in a bowl, cover with a kitchen towel and let cool for at least 10 minutes. Rub off the blackened skin, remove the stem and seeds and slice into 1/4-inch strips.
Heat a cast-iron griddle over medium-high heat on one side and medium-low heat on the other side. On the cooler side, drizzle with olive oil, add the onions and cook, stirring occasionally, until tender and beginning to char, 5 to 7 minutes. Add the roasted poblano to the onions and cook for another 3 minutes.
Sprinkle the meat with a generous amount of salt and pepper. Drizzle the hot side of the grill pan with olive oil, then lay the meat in a single layer, in batches if necessary. Cook very quickly on the first side, about 1 minute. Flip and cook on the other side, another minute or so. Repeat the process until all the meat is cooked. Reserve in a bowl along with the onions and pepper until you are ready to serve.
Heat both sides of the griddle over medium-high heat and oil lightly. Arrange the tortillas in a single layer on the griddle and heat about 30 seconds per side, then wrap in a clean kitchen towel to keep warm.
To build the tacos: In a warm tortilla, layer some meat, spoon on a couple of tablespoons of Cheese Sauce, add a few of the pepper and onions and top with the pickled cherry peppers.
Set a medium saucepan over low heat. Add the evaporated milk and Dijon and whisk together until uniform and smooth. Toss the Cheddar, pepper jack and blue cheese with the cornstarch in a bowl until totally coated. Whisk the cheese mixture into the milk mixture and heat, stirring constantly, until all the cheese is melted and the sauce is silky and smooth, about 5 minutes. Keep warm over low heat until ready to use.