Special equipment: a medium cast-iron skillet and four 5-inch cast-iron skillets
For the poblano relish: Heat the oil in a medium skillet until it shimmers. Add the onion and cook for 1 minute (just to take rawness off but still keep it crisp). Smash the garlic cloves, sprinkle with salt, and mash to a puree using the side of your knife blade. Add it to the onion along with the chiles. Season with salt and pepper and cook, stirring, to just heat through, about 1 minute. Transfer to a bowl and stir in the lime juice and cilantro.
For the Denver omelet frittatas and crispy hammy croutons: Preheat the oven to 400 degrees F. Place four 5-inch cast-iron skillets on a baking sheet and heat in the oven.
Add the oil to a medium cast-iron skillet over high heat. Add the ham, sprinkle with pepper and cook until golden brown and crispy, about 5 minutes. Remove to a plate lined with paper towels.
Add the bread to the rendered fat and cook, stirring occasionally, until golden brown and crispy, about 5 minutes. Season with salt and pepper and transfer to another plate lined with paper towels.
Whisk the eggs in a large bowl until very light and fluffy. Stir in the Monterey Jack, Cheddar and Parmesan and season with salt and pepper. Stir in the cooked ham and a few tablespoons of the poblano relish.
Carefully remove the baking sheet with the hot skillets from the oven and brush each with some softened butter. Quickly divide the egg mixture among the skillets, filling almost to the top, and immediately return to the oven. Cook until the eggs are puffy, lightly golden brown and just cooked through, about 15 minutes. Top each frittata with more poblano relish and some hammy croutons. Garnish with cilantro.