Add the beans, garlic, chipotles and salt to a blender and puree until smooth. Add stock (or water) until the puree has the consistency of a thick soup. Taste and adjust salt as needed. If you want it spicier, add a few teaspoons of the liquid from the chipotles in adobo. Transfer most of the mixture to a large shallow bowl and line a baking dish with about ⅓ cup of the mixture to create a base layer.
Warm the corn tortillas slightly over a flame, or in the microwave. Dip one side of a tortilla in the bean puree. Flip the tortilla and add a few tablespoons of onions, shredded cheese and chicken. Roll the tortilla up and place in a baking dish seam side down into the bean puree. Continue with the remaining tortillas and fillings.
Cover the tortillas in the baking dish with the remaining bean puree and sprinkle with any remaining Monterey jack cheese. Bake for 8-10 minutes or until the cheese has melted.
Serve immediately and top with queso fresco, cilantro and crema.