Cook the bacon in a large saucepan over medium-high heat, stirring, until lightly browned and crisp, 5 to 6 minutes. Add the onions and cook, stirring often and scraping the browned bits from the bottom of the pan, until soft and golden, about 10 minutes.
Meanwhile, puree the tomatoes in a blender. Add to the saucepan along with the chicken, chipotle, adobo sauce and cumin. Cook until warmed through, 1 to 2 minutes, adding up to 1/4 cup water if the sauce is too thick; season with salt and pepper.
Toss the coleslaw mix with the lime juice, 1/4 teaspoon salt and a few grinds of pepper in a medium bowl. Warm the tostadas as the label directs.
Top the tostadas with the chicken and coleslaw. Thin the sour cream with 1 to 2 tablespoons water; drizzle over the tostadas. Top with the avocado.