Green Tomatillo Salsa

Enchiladas:
Extra-virgin olive oil
1/2 medium onion, diced
3 garlic cloves, chopped
1 1/2 teaspoon ground cumin
1/4 cup all-purpose flour
2 cups chicken stock, storebought
Chopped cilantro leaves
1 deli roasted chicken (about 3 pounds), boned, meat shredded
Salt
Freshly ground black pepper
10 large flour tortillas
1/2 pound Monterey Jack cheese, shredded
2 cups sour cream
Chopped tomatoes and cilantro leaves, for garnish
Guacamole, optional