1. Combine masa harina, 1 teaspoon salt, and baking powder. Add 1 cup plus 3 tablespoons warm water (100 to 110 degrees), and quickly knead together by hand to form a smooth dough.

2. Lightly flour a work surface. Pinch off a 1 1/2-inch ball of dough, and roll it into a 4-inch disc.

3. Heat a heavy skillet, preferably cast-iron, to medium high. Cook discs, 1 or 2 at a time, until just puffed and slightly golden, 2 to 3 minutes; turn and cook 1 to 2 more minutes. Remove from heat; fold in half to create a shell. Repeat with remaining dough.

4. Heat oil in a large skillet over medium-high heat. Add onion; cook until soft, 3 to 5 minutes. Add beef; cook, stirring, until well browned, 5 to 7 minutes. Drain excess fat. Stir in tomatoes, cumin, and oregano, and season to taste with pepper and remaining teaspoon salt. Simmer until tomatoes break down and mixture is saucy, 3 to 5 minutes.

5. Fill each shell with about 1 tablespoon shredded cabbage and 1 tablespoon beef. Spoon a small amount of Pico de Gallo over beef, or serve on the side.