To poach chicken: Place the chicken in a saucepan with the onion, garlic and salt and add water to cover. Bring to a boil over high heat, reduce heat to low, cover and simmer until the chicken is cooked through, about 25 minutes. Allow chicken to cool in the poaching liquid. Remove the chicken from the saucepan. Remove skin and bones and shred the meat.
Preheat oven to 350 degrees.
Place the dried chiles in a saucepan and cover with water. Bring to a boil, lower to a simmer and cook for about 10 minutes, or until very soft. Drain. (Do NOT use this soaking liquid -- it's very bitter.)
Place the soaked chiles in a blender with half of the onion, garlic, tomatoes and 1 teaspoon sea salt. Process until smooth.
Thinly slice the remaining onion. Heat a skillet over medium heat and warm the olive oil. Add the onion and sauté for a few minutes, or until just tender. Add the chile mixture and stock to the pan and simmer 15 minutes or until flavors deepen. Taste for seasoning and add chili powder and/or sea salt to taste if necessary. Because my stock is unsalted, I always add an extra 1/4-1/2 teaspoon salt. Blend with a hand blender, if desired. This mellows out the sauce a little, but you don't have to do it.
In the meantime, warm the tortillas on a griddle or skillet on both sides until softened. Keep warm in a cotton kitchen towel.
Lightly grease the bottom and sides of a 13 x 9-inch or 12 x 8-inch baking dish with olive oil. Spoon about ½ cup of the sauce over the shredded chicken and toss. You can also mix in some shredded cheese with the chicken if you want.
Place about 1/3 cup of chicken down the middle of a tortilla and roll tightly. Place seam-side down in the baking dish. Repeat with each tortilla. Cover all the rolled tortillas with remaining sauce. Sprinkle with grated cheese, if desired. Cover and bake until heated through and cheese is melted, about 15-20 minutes. If you want the cheese to brown, put back in oven uncovered for another 5 minutes. Serve immediately with chopped cilantro, diced tomatoes, sliced green onions and sour cream or cashew cream.