Whisk the coffee, chili powder, oil, 1 teaspoon salt and a few grinds of pepper in a shallow baking dish. Add the skirt steak, turn to coat, set aside at room temperature to marinate for about 20 minutes.
While the steak marinates, brush the onion slices lightly with oil and grill, flipping occasionally, until soft and lightly charred, about 15 minutes; set aside. Drain the steak and grill, flipping once until medium-rare on the inside with deep grill marks, 3 to 4 minutes per side. Transfer the steaks to a cutting board to rest for 5 minutes and then thinly slice against the grain.
Top each tostada with a mound of steak, some grilled onions and cilantro leaves. Serve with sliced avocado, Mexican crema, crumbled Cotija cheese, pickled jalapeno, shredded lettuce and/or lime wedges.
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