Set the oven to broil, with the rack about 6 inches away from the heat source. Line a baking sheet with foil. Roast the whole poblano pepper, the whole jalapeño pepper and the onion, turning occasionally, until evenly blistered and blackened in some spots, 10 to 12 minutes. Remove from the oven and carefully gather the edges of the foil to enclose the roasted peppers and onion. Allow to cool completely, then stem, peel, seed and roughly chop the peppers, and roughly chop the onion. Set aside.
Meanwhile, lower the oven to 350º and grease an 8-inch square baking dish. In a medium bowl, whisk together the cornmeal, flour, sugar, salt, baking powder, baking soda and cumin.
In a blender, purée the remaining halved poblano and halved jalapeño, the buttermilk, cilantro, melted butter, tomatillos and egg until smooth. Transfer the mixture to a large bowl and slowly fold in the dry ingredients until just incorporated. Stir in the chopped, roasted peppers and onion along with the cheese.
Pour the batter into the prepared pan and smooth the top with a rubber spatula. Bake until golden brown and a toothpick comes out clean when inserted into the center, about 30 minutes. Let cool completely before cutting into squares and serving.