2 to 3 metal or wooden skewers soaked in water for 30 minutes
Smash the garlic cloves on a cutting board, sprinkle with the salt and with the side of a large knife, mash and smear the mixture to a coarse paste. Scrape into an 8-by-8-inch baking dish. Stir in the espresso, chili powder, oil, 1 1/2 teaspoons salt and 1 teaspoon pepper. Add the steak and coat evenly in the mixture. Cover and let marinate at room temperature for 30 minutes.
For the tortillas: Fill a large heavy-bottomed pot with about 2-inches of oil. Heat over medium to 375 degrees F. Add the tortillas, one at a time, and fry until golden brown and crispy, 30 to 45 seconds per side. Use tongs to transfer to a paper-towel-lined plate and sprinkle with salt.
Preheat an outdoor grill to high.
Push a skewer through one side and out the other of the intact onion rings (the onions should lay flat on the grill). Brush lightly with oil and grill, flipping occasionally, until soft and lightly charred, about 15 minutes. Set aside. Grill the steak, flipping once, about 5 minutes on the first side and 2 minutes on the second side for medium-rare. Transfer the steak to a cutting board and let it rest for 5 minutes. Cut the steak into 3-inch sections, and then thinly slice each piece against the grain into strips.
Assemble the tostadas:
Mound some steak onto each crisp tortilla and then top with avocado, jalapeno and grilled onions. Drizzle with some of the crema and sprinkle with cilantro leaves if using. Serve with lime wedges.
The tortillas must be fried until very crispy, otherwise they may become rubbery, making them difficult to cut with a knife and fork.
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