Prepare marinade: In bowl or gallon freezer bag, combine olive oil, lime juice, orange juice, minced garlic, cumin, chili powder, smoked paprika, salt, pepper, and cilantro. Add flank steak and coat thoroughly. Refrigerate 2-8 hours, flipping halfway through.
Preheat grill to high heat: Set to 450-500°F. For pellet grills use sear setting, charcoal grills use full chimney of coals, gas grills preheat all burners to high for 15 minutes. Clean and oil grates thoroughly.
Grill steak: Remove from marinade, let excess drip off. Place on hottest part of grill. Cook 3-4 minutes per side until internal temp reaches 125-130°F for medium-rare. Transfer to cutting board, tent with foil, rest 10 minutes.
Prepare Texas toast: Bake at 425°F for 5-7 minutes until crispy. Optional: brush edges with beef tallow before baking, then flash on grill 30 seconds to 1 minute per side for char marks.
Slice steak against grain: Identify grain direction (parallel lines running lengthwise). Position steak so grain runs left to right. Using sharp knife at slight angle, slice perpendicular to grain into ¼ to ⅓ inch strips. Let sliced steak rest 2-3 minutes.
Assemble and serve: Place 3-4 oz sliced steak on each toast. Top with cotija cheese, diced onion, cilantro, and jalapeño slices. Drizzle with chimichurri sauce. Serve immediately with lime wedges.