For the onions, cook onions, salt, sugar, and red pepper with the olive oil for 1 - 2 hours on low or medium-low until very soft but not blackened at edges. The pan should sizzle just a bit. Stir every 10 minutes or so to prevent browning on the bottom layer of onions.
When onions are caramelized, combine with the garlic and yogurt. Taste, and add salt to your preference.
For the meat, rub the steak with all the seasonings, olive oil, and Worcestershire sauce. Let sit for 30 minutes or up to overnight. When you're ready to cook the meat, preheat a skillet for at least 10 minutes to medium-high with a drizzle of olive oil.
When the pan is very hot, cook the steaks on all sides until medium or medium-rare, about 5 minutes per side for a 1-inch steak. Let the steak rest for 10 - 15 minutes, and very thinly slice against the grain for serving.
To serve, top a tortilla or taco shell with the meat, veggies, mint, cheese, and yogurt sauce.