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.
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; sauté 3 minutes. Add garlic and saute 1 more minute.
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.
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.
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.