Heat a large skillet over medium-low heat. Add 1 tablespoon olive oil and 1/2 tablespoon butter. When butter has melted and begins to foam, add the thinly sliced onion and spread into an even layer.

Season onions with 1/4 teaspoon salt and 1/4 teaspoon sugar. Cook, stirring or scraping the pan every 2–3 minutes, adjusting heat as necessary so onions brown slowly without burning. Total cooking time: 20–30 minutes until deeply golden and soft.

In the last 1–2 minutes of cooking, increase heat to medium and add 1/2 teaspoon sherry or apple cider vinegar to deglaze the pan and lift browned bits. Stir to combine and remove from heat. Keep warm.

Cream Avocado Mousse

Make mousse

Scoop the avocado flesh into a small bowl or blender. Add 3 tablespoons sour cream (or crème fraîche), 1 tablespoon fresh lime juice, 1/4 teaspoon grated garlic, 1/2 teaspoon kosher salt, 1/4 teaspoon black pepper and 1 teaspoon neutral oil.

Whisk vigorously with a fork for a slightly chunky mousse, or purée in a blender/food processor for a silky consistency. Taste and adjust seasoning — add more lime or salt if needed. If too thick, add a teaspoon of water or oil to loosen. Transfer to a piping bag or small squeeze container and refrigerate until assembly.

Tacos

Prepare beef

Pat the 12 oz of prime beef dry with paper towels and season evenly with 1 teaspoon salt and 1/2 teaspoon freshly ground black pepper.

Heat a heavy skillet or cast-iron pan over high heat until very hot. Add 1 tablespoon neutral oil and 1/2 tablespoon butter. When the fat is shimmering and the butter just beginning to brown, add the beef in a single layer. Sear without moving for 1–1.5 minutes to develop a deep brown crust, then toss and cook another 30–60 seconds until medium-rare to medium depending on thickness. Total time will vary; aim for internal temperature ~130–140°F (54–60°C). Remove from pan and let rest 2 minutes, then thinly slice against the grain if needed.

Warm the blue corn tortillas: heat a dry skillet over medium-high heat. Warm each tortilla 20–30 seconds per side until pliable and slightly charred. Keep wrapped in a clean towel to stay warm and soft.

Assemble tacos: place a warmed tortilla on a plate, pipe or spoon about 1–1.5 tablespoons of cream avocado mousse in the center. Top with half of the sliced/seared prime beef (about 6 oz total per taco). Add a generous spoonful (about 2 tablespoons) of caramelized onions on top of the beef.

Garnish with chopped cilantro (about 1/2 tablespoon per taco), 2 pickled jalapeño slices if using, a light sprinkle of sea salt flakes, and a lime wedge on the side. Serve immediately while warm.