Toss onions with garlic, chipotle powder and salt. Cook in skillet over medium-low heat until brown and caramelized. About 20 minutes. Stirring frequently. Transfer to a plate and set aside.
In the same pan, cook pulled pork just until warmed through.
Heat tostada shells according to package directions.
To assemble one tostada spread guacamole on a tostada shell. Top with 1/4 of the pulled pork, 1/4 of the caramelized onions and a dollop of sour cream and a sprinkle of cilantro. Repeat with remaining ingredients.