Combine onion and cilantro in a medium bowl and set aside.

Preheat a Carbon Steel Griddle or large Frying Pan over high. Lightly oil chile and toss in a medium bowl with salt and pepper. Place on Griddle and char on all sides, about 5 minutes total. Remove from heat and let steam in a heatproof bowl with lid until softened and skin comes off easily, about 5 minutes.

While chile steams, add bacon to Griddle and cook until crispy on both sides, about 2 minutes per side. Remove from heat. Wipe as much bacon grease off as you can with a paper towel.

Gently rub charred skin from outside of chile and thinly slice into rounds. Set aside.

Lower heat to medium-low and, working in batches, place ½ cup shredded cheese directly on Griddle in a mound. Allow to melt into a flat, round pancake shape, then crisp up (about 2 minutes), checking underneath the cheese to make sure it is golden brown and not burning.

Place a tortilla on other half of Griddle (or in another Frying Pan) to warm up, flipping when it fully puffs on each side. When cheese is fully melted, top with 1 slice bacon and 4–5 chile slices.  Roll cheese with a metal spatula, folding sides over itself, place on tortilla to form a taco, and transfer to a plate. Repeat with remaining cheese, bacon, chiles, and tortillas.  Garnish with onion and cilantro and finish with sliced avocado and a squeeze of lime.