In a bowl, combine the thinly sliced red onion, 3/4 teaspoon kosher salt, granulated sugar, and apple cider vinegar, mixing well until the sugar and salt dissolve. Let the mixture sit at room temperature for at least 1 hour to allow the flavors to meld and the onion to pickle. I find that the longer you let the onions rest, the more vibrant and flavorful they become.

In a small bowl, toss the finely shredded green cabbage with the juice from half a lime and salt to taste. Mix until the cabbage is well coated and slightly softened. This slaw adds a refreshing crunch to the tacos.

While the grill pan is preheating over medium-high heat with olive oil, season the beef tenderloin filets on both sides. Use kosher salt, black pepper, ground cumin, chili powder, and garlic powder, adjusting the quantities to your taste preferences. Make sure each filet is evenly coated with the seasoning.

Once the grill pan is hot, add the seasoned beef filets. Grill for about 5 minutes on each side for medium doneness, or adjust the cooking time depending on your preferred steak temperature. Remove the steaks from the pan and let them rest for a few minutes, allowing the juices to redistribute. Then, slice the steaks into thin strips. I like to let the steak rest for at least 5 minutes before slicing; this helps keep the meat juicy and flavorful.

While the steaks are grilling, heat the flour tortillas one at a time in an ungreased skillet over medium-high heat for about 10 seconds on each side until they are warm and pliable.

To assemble the tacos, lay out the warmed tortillas and top each with about 1/3 cup of cabbage slaw from Step 2, a few strips of sliced steak from Step 4, a drizzle of Mexican crema, some fire-roasted salsa, a sprinkle of crumbled cotija cheese, and a spoonful of pickled onions from Step 1. Serve immediately and enjoy!