Special equipment: a fondue pot
In a medium pot, heat up the chicken stock until boiling.
Meanwhile, in a medium saucepan over medium-high heat, melt the butter. Add the onions and saute until translucent, about 5 minutes. Season with a pinch of salt. Add the flour and saute briefly. Add the simmering stock to the onion mixture and whisk until there are no lumps and the stock is fully incorporated. Add the cheese and cook, whisking, until incorporated. Stir in the corn and poblanos.
Pour the mixture into a fondue pot and serve immediately with warm tortillas heated directly over a gas burner. Add cilantro leaves as a garnish and enjoy!
To char the chiles (or any fresh chile): Put the chiles over a gas flame or underneath the broiler and cook until they are blackened on all sides. Enclose them in a plastic bag and let stand for 10 minutes to steam, which will make them easier to peel.