Marinate the Steak: Whisk the olive oil, orange juice, lime juice, minced garlic, cumin, oregano, salt, and pepper together in a large dish. Add the steak, turning to coat it well. Cover and refrigerate for at least 2 hours.
Make the Creamy Avocado Salsa: Boil the tomatillos and jalapeño in a small pot of water for 5 minutes until soft. Drain them and place them into a blender with the flesh of the avocado, cilantro, lime juice, and a pinch of salt. Blend until completely smooth and creamy. Set aside.
Grill the Steak and Onions: Preheat your grill or a heavy cast-iron skillet to high heat. Brush the red onion rounds lightly with oil. Grill the steak for 4 to 5 minutes per side until nicely charred. Grill the onion slices at the same time for 3 to 4 minutes per side until they soften and show grill marks.
Rest and Slice: Transfer the cooked steak to a cutting board and let it rest for 5 minutes. Slice the meat thinly against the grain, then chop it into bite-sized pieces.
Toast the Bread: Slice your rolls in half horizontally. Spread a little butter on the cut sides and toast them face-down on your hot skillet until golden brown and crisp.
Assemble the Tortas: Spread a generous spoonful of crema onto the bottom half of the toasted roll. Pile on a hefty portion of the chopped carne asada and top it with the grilled red onions and tomato. Sprinkle a heavy layer of crumbled Cotija cheese over the meat.
