Step 1

In a large skillet, bring the onions, chiles, tomatoes, garlic, oregano, coriander, cumin, peppercorns, salt and ½ cup water to a boil over medium-high heat

Step 2

Reduce to a simmer, cover and cook on medium-low until the chiles and onions are very tender, about 20 minutes

Step 3

Remove from the heat and let cool slightly

Step 4

Transfer to a blender and puree until smooth

Step 5

Set the sauce aside

Step 6

Remove the skin from the chicken and separate the meat from the bones; discard the skin and bones

Step 7

Shred the meat into small pieces and transfer to a medium bowl

Step 8

Pour the sauce over the chicken and toss to coat

Step 9

Season with salt, if necessary

Step 10

To serve, spread refried beans over a tostada, then top with chicken and desired toppings