In a small bowl, whisk together the lime juice, minced garlic, cumin, chili powder, sea salt, and black pepper.
Place the flank steak in a shallow dish and coat thoroughly with the marinade; let it sit for at least 15 minutes to tenderize.
While the steak marinates, heat the pinto beans in a small saucepan over medium heat, mashing them with a fork until creamy and smooth.
Heat avocado oil in a cast-iron skillet over high heat until shimmering.
Sear the steak for 4-5 minutes per side until a smoky crust forms and it reaches your desired level of doneness.
Transfer the steak to a cutting board and let it rest for 5 minutes before slicing thinly against the grain.
Briefly warm the corn tortillas in the same skillet used for the steak to pick up the flavorful juices.
Assemble the tacos by spreading the mashed beans on the tortillas, then topping with sliced steak, white onion, and fresh cilantro.