Add the kosher salt, ancho chile powder, paprika, cumin, chipotle chile powder, and cayenne pepper to a small bowl and mix to combine. Place the sirloin steak into a large bowl or on a platter and sprinkle the spice mixture on top. Use your hands to thoroughly cover the steak in the spices.

Place a raised rack on a foil-lined baking sheet and place the steak on top. Refrigerate for 30 minutes or up to 24 hours.

Remove the steak from the refrigerator and preheat the oven to 300ºF. Once preheated, transfer the steak to the oven for 35 to 45 minutes. Begin checking the temperature after 35 minutes and continue roasting until the steak is a few degrees below your desired final internal temperature. See Note 2.

Remove the steak from the oven.

Heat 1 tablespoon neutral oil in a large skillet over medium-high heat until very hot. Add the steak and arrange the sliced red onion around it. Sear for 2 minutes. Flip and sear an additional 2 minutes. Transfer the steak to a wooden cutting board to rest for 5 to 10 minutes.

Reduce the heat on the onions to medium and continue sautéing for 5 to 7 minutes until softened. Add a splash of water if any residual spices in the skillet begin to burn. Turn off the heat.

Slice the steak against the grain and transfer it to a serving platter. Spoon the sautéed onions around the steak and garnish with sliced jalapeño, minced cilantro, and lime wedges, if desired. Enjoy!