Grilled top sirloin sliced and finished in a smoky, chili-spiced pan sauce, then tucked into warm corn tortillas with cotija, white onion, cilantro, and a bright chili-lime aioli. Spicy-forward by default, with easy notes for a milder, lime-forward version and a no-spice portion for guests who prefer plain steak.
Make the chili-lime aioli. In a small bowl, whisk together the mayonnaise, lime juice, lime zest, grated garlic, chili powder, smoked paprika, cayenne, and a pinch of salt until smooth and pale orange. Cover and chill until ready to serve.
Tip: zest the lime before you juice it — zesting a juiced lime is much harder.
Mix the spice blend. In another small bowl, stir together the chili powder, smoked paprika, cayenne, garlic powder, onion powder, kosher salt, black pepper, and cornstarch. The cornstarch helps the pan sauce coat the steak evenly.
Prep the toppings. Finely dice the white onion, chop the cilantro, and cut the limes into wedges. Have the cotija crumbled and ready in a small bowl.
Dry & oil the steak. Pat the sirloin dry on all sides with paper towels and lightly coat it with grapeseed oil. Dry meat sears, wet meat steams.
Build the tacos right before serving and eat them while the tortillas are still warm and pliable. Double up the tortillas (a second one underneath) for a sturdier hand-held bite, the way street vendors usually serve them.
The steak climbs a few degrees while it rests, then gets a quick second pass in the spiced pan. Going too far makes sirloin chewy.
Look for the direction the muscle fibers run, then slice perpendicular — this is what makes thin sirloin tender instead of stringy.
Cut the slices into roughly ½-inch bite-sized pieces. That's the street-taco texture and it lets the spiced sauce coat every side.
Mixing the cornstarch into the spice blend before it hits the pan makes the sauce cling to the steak instead of pooling.
Neutral flavor and a high smoke point — it lets the chili-lime profile do the talking and won't scorch on a hot grill or pan.
Reduce or omit the cayenne in both the spice blend and the aioli, then add an extra squeeze of fresh lime at serving.
Before the steak hits the spiced pan, set aside a small portion of unseasoned sliced steak for anyone who prefers no heat.
A second warm tortilla underneath catches juice and aioli and makes the taco much easier to hold without tearing.
30 seconds per side in a dry skillet or on the grill. Don't warm them too early — they stiffen as they cool.
The aioli can be mixed up to 2 days ahead and chilled — the flavors deepen and the chili powder hydrates into the mayo.
