Heat the oven to 200 C / 400 F. Blend the tomatoes with their juices, chipotles, adobo sauce, garlic, broth, oregano, cumin and salt in a blender for about 30 seconds until completely smooth.

Warm the oil in a 30 cm / 12-inch deep skillet with a lid over medium heat. Cook the sliced onion for 4 to 5 minutes, stirring now and then, until soft with golden edges.

Pour the blended sauce into the skillet and bring it to a gentle simmer. Nestle in the chicken breasts, spoon sauce over the top, cover, and cook over medium-low for 18 to 20 minutes, flipping once, until the thickest part reads 165 F / 74 C.

In the last 10 minutes of simmering, spread the tostada shells on a 33 x 46 cm / 13 x 18-inch sheet pan and bake for 4 to 5 minutes until hot and crisp. Warm the refried beans with the water in a 2-quart / 1.9 L saucepan over low heat for 5 minutes, stirring, until loose and steaming.

Shred the chicken right in the skillet with two forks. Simmer uncovered for 5 minutes, stirring, until a spoon dragged across the bottom leaves a clean trail. If sauce still pools, keep simmering, because wet tinga makes soggy tostadas.

Stir the Greek yogurt with 1 tbsp lime juice and a pinch of salt, and cut the rest of the lime into wedges. Spread about 55 g (3 to 4 tbsp) refried beans on each shell, then top with about 100 g (a generous 1/3 cup) tinga, cabbage, cotija, a spoonful of yogurt and cilantro. Serve with lime wedges and eat within 5 minutes.