Char the peppers on a barbecue grill or under the broiler with high heat until the skin is charred. Rotate the pepper and char the other side. Cook a few extra peppers in case one tears. Place the peppers in a bowl and cover tightly with foil and let them steam for 10 minutes. Carefully rub the charred skin off the pepper using a dry paper towel. Make a slit down one side and carefully remove the seeds. Do not pull on the ribs inside as that will cause the pepper to tear.
Cut the brisket across the grain in ¾" slices. Lay each slice done and cut each slice in half lengthwise. Place the cut brisket in a medium bowl and add some of the cooking liquid and stir.
Preheat 1 ½" of oil in a large saucepan over medium heat to 350°F. Preheat the oven to 200°F. Carefully spoon a heaping tablespoon of the brisket into each pepper. Add two strips of cheese and close the slit with a 6" length of a wooden skewer (they are easier to find and remove than toothpicks after cooking).
For the batter, whip the egg whites with a mixer on high speed until stiff peaks form. Beat two of the egg yolks together with the flour and salt. Then add about ½ cup of the beaten egg whites to the egg yolk mixture and stir to combine. Add the egg yolk mixture back to the egg whites and mix together. The batter will be very light and fluffy.
When the oil is hot, carefully dip a stuffed pepper into the batter. Use a spoon to coat the top, then lift the pepper by the stem and drop it into the hot oil. Repeat with a second pepper. Cook for 3 - 4 minutes then turn the peppers over and cook for another 3 - 4 minutes. Remove and drain on paper towels and place in the oven to keep warm. Work in batches to cook all of the peppers.
To plate, add ¼ cup of the sauce to the bottom of a plate. Place a cooked pepper on top of the sauce. Drizzle with Mexican Crema and garnish with pico de gallo and frizzled leeks and serve.