Pumpkin Kale and Black Bean Stew

Name: Pumpkin, Kale, and Black Bean Stew
Ingredients: 2 tablespoons extra-virgin olive oil; 2 yellow onions, diced; 5 cloves garlic, minced; 1 small sugar pumpkin, peeled, seeded, cut into 1/2-inch dice; 4 cups beef broth; 1 (16 ounce) can diced tomatoes, undrained; 1/2 teaspoon salt; 1 teaspoon ground black pepper; 1 bunch lacinato kale, stems removed, chopped; 1 pound cubed cooked ham; 1 (15 ounce) can black beans, rinsed and drained; 2 tablespoons sherry vinegar; 1 tablespoon thinly sliced sorrel