Rub the poblano and jalapeno chiles with oil and sprinkle with salt and pepper. Place on a sheet pan and roast, turning a few times, until golden brown and blistered all over, about 15 minutes for the jalapeno and 25 minutes for the poblanos. Remove to a bowl, cover with plastic wrap and let steam for 10 minutes. Remove the skin and seeds and dice or slice.
Meanwhile, melt the butter in a medium Dutch oven over medium heat, then add the flour and whisk until smooth and the raw flour taste has cooked out, about 2 minutes.
Slowly whisk in the hot milk, then bring to a boil, reduce to a simmer and cook, whisking constantly, until the mixture begins to thicken, about 7 minutes. Season with salt and pepper and the chile de arbol.
Reduce the heat to low and begin adding the cheese a handful at a time, whisking until smooth. Taste for seasoning and fold in some of the diced chiles. Pour into a fondue pot.
Mix the remaining diced chiles with the Pickled Red Onions and cilantro and top the queso with the mixture. Serve with tortilla chips, tortillas or radishes.
Pickled Red Onions:
Saute the onions in 1 tablespoon oil in a saute pan over medium-low heat until soft.
Meanwhile, place the vinegar, sugar, peppercorns, thyme sprigs and 1/4 cup oil in a noncorrosive saucepan and bring to a boil. Add the onions to the hot liquid and simmer for 5 minutes. Remove the pot from the heat and allow the onions to steep for at least 2 hours, or up to overnight. (If steeping overnight, transfer to a bowl and cover.) When ready to serve, drain the liquid and top with chopped thyme.
Use a mix of American cheese and Cheddar for a smoother consistency.