Corn Masa Pockets with Classic Shredded Beef: Gorditas con Carne Deshebrada

1 1/4 pounds boneless beef chuck steak, cut into 4 pieces 3 small white onions, diced 4 garlic cloves, peeled and finely chopped 1 tablespoon vegetable oil, plus oil to a depth of 1/2-inch for frying 1 (28-ounce) can good-quality whole tomatoes in juice, drained and chopped or 2 cups chopped ripe tomatoes 2 to 3 serranos or 1 to 2 jalapenos, stemmed, seeded and very finely chopped Salt 1 pound (2 cups) fresh, smooth-ground corn masa for tortillas or 1 3/4 cups powdered masa harina mixed with 1 cup plus 2 tablespoons warm water 1/3 cup all-purpose flour 1 scant teaspoon baking power About 1/3 cup grated Mexican queso anejo or other dry grating cheese, such as Romano or Parmesan About 1/3 cup chopped fresh cilantro, for garnish