Core and quarter 4 medium tomatoes. Place in a blender and add 3 canned chipotle peppers in adobo sauce. Blend until smooth.
Halve and thinly slice 1 small white onion (about 1 1/2 cups). Finely chop 2 garlic cloves.
Heat 1/4 cup olive oil in a large skillet or medium Dutch oven over medium heat until shimmering. Add the onion and sauté until translucent, about 5 minutes. Add the garlic and cook until fragrant, about 30 seconds. Add 1 teaspoon tomato paste and cook, stirring often, until it evenly coats the onion, 1 to 2 minutes.
Nestle 1 1/2 pounds boneless, skinless chicken thighs into the onion mixture. Sprinkle 1 teaspoon dried oregano and 1 teaspoon kosher salt over the chicken. Sauté, stirring occasionally, until the chicken is lightly browned, about 5 minutes. Add the chipotle mixture and stir to combine.
Cover and simmer, adjusting the heat as needed, until the chicken is cooked through and flavorful, about 30 minutes. Uncover and shred the chicken with 2 forks into bite-sized pieces directly in the skillet. Taste and season with more kosher salt as needed. Serve with corn tortillas or tostadas.
