Place a large skillet over medium heat and add the olive oil, 1 cup of diced onion, the oregano, cumin, and chipotle. Saute until the onion is tender; about 3 to 4 minutes. Add the garlic and cook for one minute more. Add the tomatoes and chicken stock and then slide the shredded/chopped chicken into the pan. Season to taste with salt and pepper, and allow the mixture to simmer (covered) over low heat for about 5 minutes so the flavors can settle and develop (the longer it sits the better it gets). 

To assemble the tacos, pile some of the chicken tinga mixture into tortillas (I often double up my tortillas if they’re very thin). Top with your desired toppings, such as sliced radishes, chopped fresh onion, cotija, and cilantro.