Lamb and Sweet Potato Chili

Name: Lamb and Sweet Potato Chili
Ingredients: 3 pounds lamb shoulder, cut into 1.5 to 2 -inch cubes; 3 pounds peeled sweet potatoes, cut into large (1.5-inch) cubes.; 10 garlic cloves; 1 medium onion, cut into rounds; 6 large dried pasilla chiles, stemmed and seeded; 4 large dried ancho chiles; 1 dried chile de arbol; 1 chipotle pepper in adobo (with 1 tsp of the adobo sauce); 2 teaspoons dried oregano; 1 teaspoon freshly ground black pepper; 1/2 teaspoon cumin; 3 cups homemade chicken stock; 1/4 cup diced epazote (or cilantro if you can't find epazote); 2 tablespoons vegetable oil or olive oil; 1 bottle of beer (Negro Modelo is good); Salt