Heat the remaining 1 tablespoon oil in the skillet over medium heat. Add onion, bell pepper and poblano; cook, stirring occasionally, until tender, about 6 minutes. Add garlic; cook, stirring often, until aromatic, about 1 minute. Add kale and tomatoes; cook, stirring often, until the kale is wilted, and the tomatoes are heated through, about 3 minutes. Stir in the beef and potatoes. Sprinkle with scallions, if desired.