1. Pickled Red Onion. Combine onion, vinegar, sugar, salt and 1 cup water in a small saucepan. Bring to a boil; simmer 1 minute. Pour into a heat-proof bowl, cover and chill until using.
2. Chicken Tostadas. Heat oven to 400 degrees F. Combine chili powder, cumin, coriander, cayenne and salt in a small bowl. Rub spice mixture on chicken thighs. Heat the 2 tablespoons of oil in a large, lidded saute pan on medium-high heat. Sear chicken 2 to 3 minutes per side; remove to a plate. Reduce heat to medium and add onion; saute 3 minutes. Add garlic and saute 1 more minute.
3. Pour tomatoes and broth into pan, scraping brown bits from bottom. Bring to a simmer, then return chicken to pan and reduce heat to low. Cover and simmer for 35 minutes.
4. Meanwhile, lightly brush both sides of tortillas with vegetable oil and place them in a single layer on two baking sheets. Bake at 400 degrees F for 5 minutes, flip and bake another 3 to 5 minutes, until browned and crispy. Remove from oven, cover with aluminum foil and set aside. Remove chicken, shred and return to pan; discard bones.
5. To assemble: Place one tortilla on plate. Transfer a bit of chicken mixture to tortilla using a slotted spoon. Place another tortilla on top. Garnish with Pickled Red Onion and, if desired, cheese and cilantro.