In a large saute pan over medium high heat, add half of the oil and minced onion. Let cook for 3 minutes then add ground chicken, minced garlic, oregano and cumin. Cook and crumble until no longer pink.
On a parchment paper covered baking sheet add a baking rack and spray with cooking oil. Place halved tomatoes and tomatillos as well as remaining half of the white onion. Drizzle with remaining oil. Place in oven to roast for 20 minutes.
In small sauce pan over medium-low heat, add refried beans and taco seasoning. Stir to combine. Continue to cook and stir occasionally until ready to build tostadas.
Remove vegetables from oven and add to bowl with entire can of chipotle in adobo. Blend until smooth with immersion blender. (You can also use a blender). Add mixture to chicken. Toss to coat and reduce heat to medium low. Let simmer for 10 minutes.
Add tostadas to baking sheet in a single layer. Place in oven for 5 minutes, then remove.
To build a tostada, smear with portion of refried beans. Add a large spoonful of chicken tinga then add a dollop of sour cream. Sprinkle with queso fresca and cilantro. Repeat with remaining tostadas.