Add boneless, skinless chicken thighs to slow cooker with garlic, Worcestershire sauce, Dijon mustard, beef bone broth, and rosemary sprig
Cook on High for 3-4 hours OR Low for 4-5 hours
Cook diced yellow onions over high heat until they release all their water and become caramelized
Pour beef bone broth, Worcestershire sauce, and Dijon mustard over caramelized onions and boil over high heat until water evaporates
Remove chicken from slow cooker and break it up with a metal spoon
Combine cottage cheese, Gruyère, Parmigiano Reggiano, and about three quarters of the caramelized onions in a blender or food processor to make cheese sauce
Add approximately 1 cup of pasta water to the slow cooker
Add half-cooked rigatoni pasta to the slow cooker
Pour cheese sauce over the pasta and chicken mixture
Cover and cook for 15-20 more minutes until pasta finishes cooking
Top with chopped chives and remaining caramelized onions before serving
