In a large bowl, whisk together lime juice, minced garlic, cumin, salt, pepper, and vegetable oil.
Add the skirt steak to the marinade, coating thoroughly, and let sit at room temperature for 15 minutes.
Preheat a grill or heavy cast-iron skillet over high heat.
Sear the steak for 4 to 5 minutes per side for medium-rare doneness.
Transfer the steak to a cutting board and allow it to rest for 5 minutes.
Slice the steak against the grain into thin strips, then cut into bite-sized pieces.
Spread approximately 3 tablespoons of warm refried beans onto each tostada shell.
Layer the sliced carne asada over the beans.
Top with shredded lettuce, cotija cheese, pico de gallo, avocado slices, and a drizzle of Mexican crema.