Fill a large, deep skillet halfway with vegetable oil. Heat the oil over medium-high heat until a deep-fry thermometer registers 350 degrees F. Place a cooling rack over a baking sheet. Fry the tostones in 2 batches for about 2 minutes per side, or according to package directions. Drain on the cooling rack.
Dice the sirloin (do this when it's cold right out of the refrigerator so it's easier to cut into small pieces). Place the steak in a medium bowl. Add the onion and mojo sauce and toss. Season generously with salt and pepper.
Mash the avocado, lime juice and a pinch of salt together in a large bowl. Set aside.
Heat a medium cast-iron skillet over medium-high heat. Add the olive oil. Use a slotted spoon to transfer the steak and onions to the skillet, leaving the liquid in the bowl. Sear the steak until the exterior is seared, 2 to 3 minutes. Add the reserved mojo to the pan and cook until steak and onions are browned and caramelized, 3 to 4 minutes more. Turn off the heat, then season with salt and pepper.
The tostones will vary in size, so match up sets of two that are approximately the same. Spread a thin layer of avocado on one side of the tostones.
Build sliders by layering one tostone with steak, pico de gallo, a pinch of potato sticks and cilantro and top with a second tostone that is about the size of the first. Repeat with remaining tostones.