For the sauce, add the sliced roma tomatoes, chipotles in adobo, garlic, chicken stock, and salt. Blend in a blender until smooth.
In a large pan with a lid, add oil over medium heat. Add sliced yellow onions and cook until translucent and some caramelization. This takes about 10 minutes.
Add cooked shredded chicken to pan, then pour tomato chipotle sauce. Mix well and bring to a low boil, cover, and simmer for 10 minutes. Taste and see if additional salt or pepper is needed.
To assemble tostadas- spread refried beans on the bottom, top with chicken tinga, shredded lettuce, pickled red onions, crumbled cheese, and sliced avocado.
Recipe for Homemade Re-fried Beans, click Here.Homemade Tostadas: place tortillas directly on the oven rack or toaster oven at 375 degrees for about 7-10 minutes or until crispy.Pickled Red Onions: Slice red onions and place in a glass jar with a lid. Fill halfway with fresh squeezed lime juice and half teaspoon of salt. Close, lid, shake well, and place in fridge for at least 30 minutes or up to 1 week.