In a small pan over high heat, cover red onion with cold water and bring to a boil. Remove from heat and drain. Put red onion in a small bowl with vinegar, 1/2 tsp. salt, and pepper. Set aside to marinate at least 30 minutes at room temperature or up to 1 week covered and refrigerated.
To a small frying pan over high heat, add 1 in. of vegetable oil and heat to 375°. Fry tortillas, one at a time, until golden and crispy. Drain on paper towels.
Heat olive oil in a large frying pan over high heat. Add sliced white or yellow onion and 1/4 tsp. salt. Cook, stirring, until onions are soft, about 4 minutes. Add oregano and cook until fragrant, about 1 minute. Add turkey and broth. Bring to a boil, lower heat to a steady simmer, and cook, partially covered, for 10 minutes to let flavors blend.
Meanwhile, in a small bowl, combine chiles, lime juice, orange juice, and remaining 1/4 tsp. salt. Set aside.
To assemble: Divide turkey-onion mixture among the 8 tortillas, then top each with avocado, cabbage, and drained pickled red onion (you will have extra onion; see Notes). Serve with serrano-citrus sauce.
Note: Nutritional analysis is per serving.