Coat 6-quart slow cooker with cooking spray.
In a large pan, heat olive oil over medium-high heat. Season the beef with cumin, salt and pepper. Add to the pan and cook for about 3 minutes on each side, until browned.
Place meat in the slow cooker. Add onions, garlic, and salsa verde. Cover and cook over low heat for 8 hours.
Shred the meat with 2 forks. Set aside.
Preheat oven to 350 degrees F. Prepare a 9-inch by 13-inch pan by lightly spraying with cooking oil. Set aside.
Prepare the white sauce: In a saucepan over medium heat, melt the butter. Add the flour and whisk constantly for one minute. Add the chicken broth and continue to whisk until smooth. Stir in the sour cream and remove from heat.
Note: If you like, you can begin preparing the sauce about 15 minutes before your beef is finished cooking. Otherwise, it doesn't take long to prepare the sauce.
To assemble enchiladas, fill each tortilla with beef, roll and place in casserole dish seam side down. Pour white sauce over the top of the enchiladas. Bake for 20 minutes.
To serve, you can optionally pour some additional salsa verde over the enchiladas and sprinkle with chopped cilantro.