Position racks in the upper and lower thirds of the oven; preheat to 375°F. Bring tomatoes and their juice to a boil in a medium saucepan over medium heat. Add onion and cook, stirring occasionally, until the onion is soft and most of the liquid has evaporated, 15 to 20 minutes. Add turkey (or chicken) and cook until heated through, 1 to 2 minutes. Meanwhile, coat tortillas on both sides with cooking spray. Divide the tortillas between 2 large baking sheets. Bake, turning once, until crisped and lightly brown, about 10 minutes. Mash avocado in a bowl. Stir in salsa, sour cream and cilantro until combined. To assemble tostadas, spread each crisped tortilla with some of the avocado mixture. Top with the turkey (or chicken) mixture, lettuce and cheese.