How to Make It
To make the salsa, light a grill or preheat a grill pan. Grill the tomatoes, halved onion and garlic over moderately high heat, turning, until charred in spots, about 10 minutes. In a saucepan, simmer the tomatoes, onion, garlic and chipotles with 1 1/2 cups of water over moderate heat until the onion is softened and the tomatoes burst, about 30 minutes. Transfer to a blender and puree until smooth. Season the salsa with salt and pepper.
Roast the poblanos over a gas flame, turning, until charred. Transfer to a bowl, cover with plastic and let steam for 15 minutes. Peel, seed and stem the chiles, then dice.
Preheat the oven to 425°. Layer half of the chips on a foil-lined rimmed baking sheet. Scatter half of the cheese, beans and diced poblanos over the chips. Repeat the layering with the remaining chips, cheese, beans and poblanos. Drizzle half of the salsa on top and bake for about 10 minutes, until the cheese is melted. Drizzle crema over the nachos, top with guacamole and the remaining salsa, then sprinkle with cilantro and minced onion. Serve.