Preheat oven to 375 degrees F. Brush a thin layer of canola or vegetable oil on both sides of each flour tortilla. Place a 4-inch ramekin face down on a rimmed cookie sheet and drape one tortilla over it. Press heavy-duty aluminum foil over the top and shape into a bowl. Flip open-end up and repeat for remaining tortillas.

Bake tortilla bowls for 5 minutes. Carefully remove ramekins with tongs and return tray to oven for 5 more minutes until edges begin to turn golden.

Remove from oven. Lift each shell out of the foil and place bottom-side up on the cookie sheet. Tuck foil loosely around the rim of each shell to prevent over-browning. Return to oven for 7 to 10 more minutes until fully crisp and golden. Set aside to cool slightly.

Add ground beef to a large skillet over medium heat. Break it up with a wooden spoon and cook for 7 to 8 minutes until no pink remains. Drain off the grease. Add taco seasoning and water, stir to coat, and cook for 5 minutes until the liquid reduces and seasoning clings to the meat. Stir in black beans and corn, then remove from heat.

Place a tortilla bowl or a handful of tortilla chips in the bottom of a shallow bowl. Add a generous layer of chopped romaine. Spoon on the warm taco meat mixture. Add desired toppings and finish with salsa, sour cream, and guacamole or fresh avocado. Serve immediately.