Preheat oven to 350°F. Roast chiles and garlic on a rimmed baking sheet until lightly browned and softened, 12–15 minutes. Add cashews to baking sheet and roast until nuts are golden brown, 5–7 minutes. Let cool slightly, then peel garlic. Transfer garlic, cashews, and chiles (include all the seeds if you like things hot, or half of them if you aren’t sure) to a blender; add vinegar, sugar, and 1/2 cup water and purée until smooth. Season with salt.
Slaw and assembly:
Whisk vinegar, mayonnaise, and yogurt in a medium bowl. Add cabbage and scallion and toss to coat; season with salt. Set aside.
Season steak with salt and pepper; sprinkle with cumin. Heat 1 Tbsp. oil in a large skillet over medium-high. Cook steak until deeply browned, about 4 minutes per side for medium-rare. Transfer to a cutting board; let sit 10 minutes before slicing.
Meanwhile, if making your own tostadas, pour oil into a small skillet to 1/4\" depth. Heat over medium. Working one at a time, fry tortillas until crisp, about 1 minute per side. Transfer to paper towels.
Top tostadas with some salsa, followed by steak, more salsa, slaw, and cilantro. Serve with lime wedges.