Rub the flank steak on both sides with salt and pepper. Heat the vegetable oil in a heavy skillet on high heat and cook the steak on one side for 5 minutes. Flip and cook on the other side for 5 more minutes for medium-rare. Keep warm.
Meanwhile combine the red onion, cucumbers, tomatoes, avocados, cilantro, mint, lime juice, and olive oil in a bowl and mix well. Season the salad with salt, and pepper, to taste and set aside.
For the jalapeno poppers:
Heat oil in a deep fryer or a heavy-bottomed skillet to 350 degrees F.
Carefully slice down the side of the peppers and remove the seeds. Set aside.
In a bowl, combine all the grated cheeses, season with pepper, to taste and stuff the peppers. In a separate bowl, mix the flour, egg, and beer to form a batter. Dip the stuffed peppers in the batter and fry in a deep fryer or in a heavy skillet for about 5 minutes or until golden brown and crispy.
Slice the flank steak. Pile the salad on a plate with slices of flank steak, and top with 1 popper, sliced in half.