3/4 lb. yellowfin or albacore tuna, cut in 3/4-inch cubes
1/2 peeled avocado, diced, for garnish
1/4 cup chopped scallion, for garnish
16 tortilla chips, crumbled, for garnish
Roast the poblano pepper over an open flame until its skin blisters. Seal it in a paper bag to steam for 10 minutes. Using your fingers, slip off the skin. Seed and chop the pepper. Place it in a blender. Add the garlic, canned chiles and cilantro. Set aside.
Heat a heavy skillet over medium-high heat. When hot, add the onion slices in one layer. Cook until grilled in places, 1 minute. Turn and grill on the other side, 1 minute. Transfer to a plate. Pan-grill the tomatillos, cut side down, until dark brown in places, about 2 minutes. Turn and cook until they brown in spots on the rounded side, 2 minutes. Transfer them to the plate.
Coarsely chop the tomatillos and onions. Add them to the blender along with 1 cup of the broth. Process into a pulpy purée. Transfer to a deep saucepan. Stir in remaining broth. Season to taste with salt. Over medium heat, bring to a boil. Transfer to 4 shallow soup bowls.
Rinse and dry the skillet. Add the oil and heat over medium-high heat. Add the tuna chunks, searing and turning them until they are opaque all the way through, about 3 minutes. Divide the fish among the bowls. Garnish with avocado, scallions and tortilla chips. Serve.