"The best solution to Thanksgiving leftovers...."
butter, divided, plus more for pan
yellow onion, finely chopped
cloves garlic, minced
all-purpose flour
Dijon mustard
whole milk
low-sodium chicken broth
shredded white cheddar
sour cream
cayenne pepper
Kosher salt
Freshly ground black pepper
cooked turkey breast, diced
frozen broccoli, thawed and drained
crushed Ritz crackers
Freshly chopped chives, for serving