Whisk together the sour cream, mayonnaise, and one tablespoon taco seasoning together in a medium bowl and set aside.
Place the top oven rack in the second position from top and pre-heat broiler to “High.” Line a large, rimmed baking sheet with aluminum foil and set aside. Place a large 12” cast iron skillet over medium-high heat.
Place the flank steak on the prepared baking sheet and evenly brush the olive oil over both sides. Evenly sprinkle one tablespoon taco seasoning over the steak. Generously season with sea salt before turning. Repeat on the other side with the remaining taco seasoning and a little more salt.
Place the steak under the broiler for 5-7 minutes, turning once halfway through, or until it reaches the desired level of doneness. Tip: See this chart for recommended internal temperatures for beef. While the steak is broiling, add two corn tortillas to the hot skillet. Heat until they are warmed through and develop a bit of color, approximately 30 seconds to one minute. Flip with tongs and repeat on the remaining side. Transfer the warmed tortillas to a large sheet of aluminum foil (or a tortilla warmer, if you have one). Repeat with the remaining tortillas. Wrap the tortillas in the aluminum foil while working on the rest to keep warm.
When ready, remove the steak from the oven and let rest for 5 minutes before cutting against the grain into thin slices. Transfer the steak to a medium bowl and toss with the fresh lime juice. Set aside. Tip: Continue warming the rest of the tortillas while the steak rests.
To serve, layer two tortillas together, as shown, and add some of the seasoned steak and your choice of toppings, such as sliced radishes, crumbled Cojita cheese, diced avocado, or fresh cilantro. Top each taco with a dollop of the seasoned sour cream mixture and serve immediately with additional lime wedges on the side.