Preheat the oven to 350°F and grease a 9x13 inch casserole dish with nonstick cooking spray.
In a large deep skillet, sauté chopped onion in butter over medium-low heat until soft and translucent.
Sprinkle the flour over the onions, stir and let cook for a minute.
Slowly add the chicken broth while constantly stirring or whisking.
Bring the sauce to a boil over medium heat and boil for 1 minute and sauce coats spoon.
Remove skillet from heat and stir in the sour cream.
Remove 2 cups of the sauce and set it aside.
Stir the salsa verde into the remaining sauce in the skillet and set aside.
In a large mixing bowl, combine the drained and rinsed cannellini beans, cooked shredded chicken, ground cumin, half of the shredded cheese and the reserved 2 cups of sauce.
Scoop about a heaping ¼ cup of chicken mixture into the center of a tortilla. Fold in the sides and roll. Place enchilada seam side down in prepared baking dish. Repeat with remaining mixture and tortillas.
One all the enchiladas are in the baking dish, pour the salsa verde sauce evenly over the top of the enchiladas.
Sprinkle the remaining shredded cheese on top of the sauce and enchiladas. Bake the enchiladas uncovered for 30-35 minutes until bubbly and the cheese is melted.
Garnish with fresh cilantro or parsley, crumbled Cotija or Queso Fresco cheese and serve.