Heat olive oil in a saucepan over medium heat. Add the onions; cook and stir until softened and translucent, about 5 minutes. Meanwhile, puree the tomatoes with chipotle peppers and adobo sauce to taste. Pour into the onions, and add chicken. Cover, and simmer for 20 minutes.
To serve, mound the chicken onto tostada shells, and garnish with a dollop of sour cream.