Preheat an oven to 375 degrees F (190 degrees C). Grease a large muffin pan with cooking spray.
Combine ground beef, blue cheese, onion, bread crumbs, spinach, eggs, and Worcestershire sauce in a large bowl until well blended. Divide meat mixture evenly into the prepared muffin pan.
Bake in the preheated oven until no longer pink in the center, about 30 minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C).
