Add the chuck roast cubes, chopped onion, minced garlic, taco seasoning, and salsa verde to the slow cooker insert.
Cover and cook on low for 8 hours.
Remove the cooked beef.
Carefully remove all but 1/2-1 cup cooking liquid, depending on desired sauciness.
Return the beef and the reserved cooking liquid to the slow cooker insert.
Shred the beef into the the cooking liquid, and serve.
Notes
Beef: Beef stew meat can be used in place of the cubed chuck roast.
Onion: Yellow onion or sweet onion will also work.
Taco seasoning: Store-bought or homemade taco seasoning will work.
Salsa verde: Mild or medium salsa verde works well.
To freeze, add all of the ingredients to a freezer-safe container. Freeze for up to 3 months. To use, thaw in the fridge before using. Add the thawed contents of the freezer container to the slow cooker. Cook as directed.