• 12 (6-in.) corn tortillas
• 1 c. finely chopped onion, divided
Toast anchos and cumin seeds in 12-inch skillet over medium heat, stirring constantly, until fragrant, about 1 minute. Transfer to a spice grinder (or use a blender in the "grind" mode) and let cool for 5 minutes. Add peppercorns and oregano, and process until finely ground, about 30 seconds.
Heat 3 tablespoons oil in now-empty skillet over medium-high heat until shimmering. Add chile mixture, garlic powder, flour and 1/2 teaspoon salt and cook, stirring constantly, until fragrant and slightly deepened in color, about 1 minute.
Slowly whisk in broth, scraping up any browned bits and smoothing out any lumps. Bring to simmer and cook, whisking often, until sauce has thickened and measures 1 1/2 cups, about 5 minutes. Whisk in vinegar and season with salt and pepper to taste. Remove from heat and cover to keep warm.