Prepare the marinade by whisking together lime juice, olive oil, minced garlic, cumin, oregano, salt, and pepper in a small bowl.
Marinate the flank steak in the mixture for 30 minutes in the refrigerator.
Heat a grill or heavy skillet over medium-high heat.
Cook the steak for 5 to 7 minutes per side until an internal temperature of 135°F (57°C) is reached for medium-rare.
Remove steak from heat and let rest for 5 minutes before slicing against the grain into 1/2-inch cubes.
Spread 1/4 cup of warm refried beans onto each tostada shell.
Layer each tostada with shredded lettuce and a portion of the grilled steak.
Garnish with crumbled cotija cheese, avocado slices, pico de gallo, and a drizzle of Mexican crema.