How to Make It
In a large saucepan, heat 2 tablespoons of olive oil until shimmering. Add half each of the onion and garlic and cook over moderate heat until they are softened, about 6 minutes. Add the tomatoes and their liquid and cook the sauce over moderate heat for 25 minutes.
Light a grill or preheat the broiler. Rub the poblanos and habanero lightly with olive oil and grill or broil, turning frequently, until the skins are charred but the chiles are still firm, about 3 minutes. Transfer the charred habanero to a plate and set aside. Transfer the charred poblanos to a bowl, cover with plastic wrap and let stand for 15 minutes, or until cool.
Peel the poblanos, being careful not to rip the flesh. Make a lengthwise slit down one side of each poblano and carefully remove the core and seeds.
Working in batches, puree the tomato sauce until smooth. Return the puree to the saucepan and add the whole, unpeeled charred habanero. Simmer the sauce over moderate heat, stirring occasionally, until very thick, about 40 minutes; be careful not to break the habanero or the sauce will be extremely spicy. Remove the habanero and discard. Season the tomato sauce with salt and pepper.
Meanwhile, in a large, deep skillet, heat the remaining 2 tablespoons of olive oil until shimmering. Add the remaining onion and garlic and cook over moderate heat, stirring occasionally, until softened, about 6 minutes. Add the ground sirloin and cook, breaking it up with a wooden spoon, until no trace of pink remains, about 8 minutes. Add the olives, capers, currants, cumin and water and season with salt and pepper. Cook the sirloin mixture over moderate heat until the liquid is nearly evaporated, 2 to 3 minutes. Let cool.
Preheat the oven to 400°. Spoon 1 cup of the tomato sauce into a 9-by-13-inch glass or ceramic baking dish. Stir 1/4 cup of the tomato sauce into the beef filling and season with salt and pepper. Stuff each poblano with about 2/3 cup of the beef filling and carefully arrange the chiles in the baking dish. Cover the dish with foil and bake for about 15 minutes, until heated through. Sprinkle the stuffed poblanos with cilantro and serve right away, passing the extra tomato sauce at the table.